Malcolm, a 15-year-old at a city high school, was asked by his mother last week if he knew where to get contraceptives. "It was the first time my mom has asked me things like that," said the lanky teen-ager. "I think she was worried about the new law."

Wisconsin lawmakers have given parents an urgent reason to hold long-avoided talks with their children about sex. Thanks to a month-old law -- designed in part to force parents to become more responsible for their children's sexual behavior -- they are now required to foot the bill if their teen-age son or daughter becomes a parent.

Under Wisconsin's new law, the grandparents of an infant born to unmarried minors can be forced by court order to support the baby until its parents turn 18, get married or join the armed services.

The grandparents' liability law was part of a 10-part legislative package that united pro- and antiabortion forces in an unusual show of support. Among other things, the law imposes new criminal penalties on those performing late-term abortions, repeals an obsolete statute imposing criminal penalties against women seeking abortions, removes prohibitions on the display and advertisement of contraceptives and establishes a state adoption center. (The taboo against sale of contraceptives in vending machine remains. "Vending machines would encourage more casual sex, in taverns and dance halls," said Chuck Phillips, director of the Wisconsin Catholic Conference.)

In a country where sex sells everything from telephones to cars and where teen-age pregnancy has reached epidemic proportions, Wisconsin is trying to fight back with an ambitious social experiment.

The goals are social and financial -- to cut the number of children (about 2,400 in 1983) born to parents under the age of 18, to reduce the need for some of the 20,000 abortions performed in the state each year and to trim the state's welfare rolls, which currently support 285,000 women and children.

"We had to kick parents in the pants because that's where their wallets are," said State Rep. Marlin Schneider (D), father of the Abortion Prevention and Family Responsibility Act of 1985. "Before this, the taxpayers have been shouldering all the freight."

The jovial former schoolteacher, who is also known as "Snarlin' Marlin" for his brashness, won a standing ovation from his colleagues on the night the bill passed. The acclaim was generated by the unprecedented discovery by pro- and antiabortion forces of common ground among the emotional minefields of sex and abortion. Although both sides are uneasy about portions of the law, neither side wanted to be blamed for derailing the compromise.

"It's a rather creative law that's caught the imagination of the people of Wisconsin," said Barb Luchsinger, a spokeswoman for Planned Parenthood of Wisconsin. "It doesn't try to legislate morality; it is an attempt to try education and communication."

Barbara Lyons, chief lobbyist for Wisconsin Citizens Concerned for Life, an antiabortion group, said her group grudgingly agreed not to oppose the law because of the new criminal penalties (a two-year prison term and a $10,000 fine) for those performing late-term abortions. Pro-choice forces won an end to criminal penalties against women seeking abortions, contained in an unenforced and unconstitutional law that antiabortion groups once hoped would be reactivated in the event of an overturning of the 1973 Supreme Court ruling legalizing abortion.

Counselors at local family-planning clinics said the law already is having an impact. "We have been seeing young men come in asking for condoms," said Allison McEwen, a clinical assistant at a Madison family-planning center. "They said their fathers told them to get them. This is unusual for us; we generally see only the girls and their mothers."

In one newspaper cartoon, a father says to his son, "I think it's time to explain the facts of life!" His son asks, "You mean, like the birds and the bees?" The father answers, "Ah, actually, it's more like the bucks and the dough."

The grandparent liability law has grabbed most of the attention. Catholic lobbyists opposed the provision, on the theory that grandparents would bring new pressure on girls to abort. As a result, this section is the only one with a set expiration date, in 1989.

But the provision appealed to taxpayers fed up with welfare costs and angry at parents they believe are permissive and irresponsible.

By next year, Wisconsin officials say they will be able to determine whether the law works and whether it produces answers that can be applied elsewhere. According to federal estimates, more than a half million babies will be born this year to unwed teen-agers in America. Ten thousand of these mothers will be under the age of 15.

Many wonder how the law can control something as unpredictable as a teen-ager's behavior. A recent Louis Harris poll found 64 percent of the parents polled said they had little or no control over their teen-ager's sexual behavior and said they needed help from an outside source to educate their children.

"Can money conquer sex?" asked Eleanor Yeo, director of the Milwaukee Women's Health Organization, an inner-city abortion clinic. "Kids aren't ever going to ask their parents before they have sexual intercourse."

She added, "How can this law counter all the songs and movies and famous stars having out-of-wedlock babies? . . . What we need are movies on how tough it is to be a teen-age parent. What we get is Paul Anka singing about 'having my baby' and how wonderful it is."

Under the law, both sets of grandparents must support their grandchild if their child cannot. If grandparents can't work out an agreement informally, a local district attorney can take the case to court. The amount of support varies according to a grandparent's ability to pay, with the richer ones paying more.

Observers worry that teen-age fathers may contest paternity in order to delay proceedings past their 18th birthday. Although a sophisticated blood test pinpoints paternity in nearly all cases, most health authorities wait until an infant is six months old before withdrawing the necessary blood.

"Most of us feel there won't be a substantial number of cases," said Tom Kaplan, an analyst with the state's Division of Policy and Budget. "It could lead to fewer pregnancies and some extra collections, but the main importance is an expression of social responsibility."

One reason the law may have limited effect on the state's $219 million annual welfare bill is that many parents of teen-age parents are on welfare. "Very few teens who are pregnant come from intact families," said clinic director Yeo. "There's usually not much money there to go after. And if the law means there will be a delay in getting AFDC [Aid to Families With Dependent Children] to these children, it will be a terrible thing."

All of these uncertainties about the law will not be settled until regulations are written. Currently, social workers taking applications at welfare offices are telling clients about the law, but are not enforcing it yet, according to Linda Reivitz, Health and Social Services commissioner for the state.

In January, a seven-member Pregnancy Prevention board will start doling out $1.7 million in grants to cover the next two years. The money will support health clinics in schools, a media campaign to discourage teen sex, adoption counseling and role-model programs. The grant money supplements the $1.5 million yearly the state currently spends on teen pregnancy programs, according to Reivitz.

"This should make a big difference," Reivitz said. "Wisconsin, for all its liberal tradition, has been ignoring teen pregnancy. Who believes that 13-year-olds have babies? Who believes there are 28-year-old grandmothers?"

Teen-agers at Malcolm Shabazz High School, an alternative high school in this city, say the law will help many of their friends who are unable to escape welfare after having a baby or who have sex without protection.

"Everyone says they want to take care of the baby themselves," said Cindy Dorman, 16. "Not on a $3.35 an hour job. It's a trap; you can't go to school and you can't work . . . . It's so important, there should be classes for parents who have a hard time talking to their kids about sex."

Nikki Wegner, 15, added, "A lot of people chicken out about getting birth control. They're scared someone will see them going into Planned Parenthood and tell on them." The law does not allow contraceptives to be dispensed in schools but does encourage family planning to be taught, an important consideration in a teen-age society, where, according to Wegner, "if you're 16 and still a virgin, people laugh at you."

Legislators were careful not to move too fast. Although all 432 public-school districts now must set up committees to draw up a sex-education curriculum, nothing requires the schools to teach the classes.

"Our hope is there will be a certain momentum and schools will go ahead and use the materials," said Luchsinger of Planned Parenthood.

The classes, which currently are taught in 40 percent of Wisconsin's schools, avoid mention of sex in their title. They are "Human Growth and Development Classes," a legitimate name, supporters say, because they include classes on self-esteem, moral responsibility and other themes.

Arnie Chandler, head of the curriculum bureau of the Department of Public Instruction, notes that parents will still have the right to remove their children from the classes, "if for philosophical or religious reasons they object to what's being taught."

The new legislation stemmed from the work of a study commission last summer, set up after lawmakers deadlocked on whether to ban abortions at public hospitals. Despite several stormy hearings and predictions of failure, the measure contained sufficient compromises to win accord.

Schneider, the father of two daughters, ages four and seven, admits that he, too, will find it hard to overcome traditional parental discomfort when dealing with his children's sexuality.

"If my kids are going to be sexually active, I'd rather just let my wife take care of it," he said, blushing. "When I was younger and it was with another man's daughter, sex was one thing. When it's your own daughters, it's another thing entirely."