The tobacco companies have had an awesome winning streak over the decades, never paying a penny to a smoker while fending off more than 200 product-liability lawsuits that blamed cigarettes for disease, injury or death.

That record is now facing its severest challenge in two pending wrongful-death cases: One that goes to trial tomorrow in tiny Lexington, Miss., in the heart of one of the nation's poorest counties, and the other on Jan. 19 in Newark. Yet even if the companies win in both courtrooms, they will face a barrage of unprecedented attacks on allegedly toxic pesticides and additives in cigarette tobacco, and on advertising and other industry practices.

In the Mississippi case, the family of the late Nathan H. Horton seeks a compensatory award of $2 million and punitive damages of $15 million from American Tobacco Co. (ATCO), a subsidiary of American Brands Inc. of Old Greenwich, Conn.

Lawyers for both the plaintiffs and the defense refuse to comment on the case. But papers on file in Holmes County Circuit Court make it clear that the plaintiffs' lawyers will try to focus the jury's attention on the conduct of a cigarette company and on the contents of its products and away from the defense claim that a person smokes as a matter of free choice.

In a key victory for the family, Judge Gray Evans ruled on Dec. 15 that Mississippi's wrongful-death act applies to the case. This law allows an injured person to sue for alleged violation of a manufacturer's implied warranty of "the purity or fitness of any ... tobacco ... intended for human consumption ... " Horton's lawyers claim ATCO violated the law with an implied warranty of the purity of Pall Mall, the brand Horton smoked regularly for 32 years before he died of inoperable lung cancer last Jan. 27, when he was 50.

In a sworn statement supporting this argument, a materials expert who analyzed the tobacco and paper of a single nonfiltered Pall Mall said he found 46.1 million tiny fibers, including inorganic fibers of asbestos, cadmium and other substances that "do not naturally appear in tobacco." The expert is William E. Longo, president of Micro Analytical Laboratories Inc. in Gainesville, Fla., and a member of an Environmental Protection Agency advisory group. Longo did not address the health implications of his findings.

One defense witness who might challenge such testimony is Richard Thompson of the University of Alabama, an environmental health specialist. But ATCO put his name on its list of expert witnesses well after the deadline set by Judge Evans, and it is not certain if he will testify. In general, the company's witness list indicates that a major defense will be that the cancer cells found in Horton's lungs were of a particular type that tobacco smoke hasn't been shown to cause.

In another affidavit, Walter W. Dickerson, 58, a maintenance man who worked for 32 years at the Pall Mall plant in Reidsville, N.C., told of a pesticide that was sprayed regularly at night "on all tobacco used to make cigarettes in the plant."

He was unable to name the pesticide, but said it came in barrels "marked with skull and crossbones." When it "dripped on metal railings or concrete floors it would eat them up," he said. "I saw where it had happened."

"The tobacco which had been sprayed was not washed off or otherwise cleaned to get the spray off," Dickerson swore. "The tobacco with the residue from the spray was made into cigarettes."

He left in 1979 after "I was sprayed on several occasions with no notice. ... I can no longer see except for outside vision. I am now drawing retirement and Social Security disability."

ATCO has said in a court paper that Dickerson "is not a scientist and has no educational background in chemistry," and is "motivated by an animus" against ATCO.

Other troubling questions beset defense counsel, led by the New York law firm of Chadbourne & Parke. One is how the jurors to be chosen tomorrow will assess Robert K. Heimann, who retired in 1980 as chairman and chief executive of both American Brands and ATCO, and who has been a smoker since his early teens. Pall Mall is his favorite brand.

The jury will view the videotaped testimony by two men from very different worlds: Heimann, the Princeton-educated, retired CEO of a giant corporation, and Horton, a former Navy enlisted man who "born by a midwife" in the home he and his family built with their own hands just outside of Lexington.

Heimann testified that ATCO's position that smoking doesn't cause disease was formulated principally by a lawyer, former CEO Paul Hahn, who did not consult with physicians. During Heimann's quarter-century at the helm, ATCO convened public-relations experts -- but never doctors -- to get opinions on smoking and health, he testified.

Heimann, who has advanced degrees in sociology and statistical analysis, declared himself more qualified than a physician to interpret "statistical nuances" relating to the risks of smoking. Similarly, he denied that the Surgeon General is more qualified than he to determine whether smoking is hazardous, and pronounced wrong all of the leading health organizations that claim smoking is hazardous.

The plaintiffs contend that Pall Mall advertisements violated an express or implied ATCO warranty that smoking the cigarettes was "wholesome and safe." One of the ads said against throat scratch." Heimann said the ads associated Pall Malls with fun, love, "happy occasions," outdoor activities, "pleasure" and "satisfaction."

The issues of product purity and promotion and of Heimann's views on smoking and health are not the end of ATCO's problems in the upcoming trial.

In many states, for example, a jury can't return a verdict for a plaintiff if it finds him more than 50 percent responsible for his injuries. Mississippi became the first state to enact a pure comparative-fault law that weighs the fault of each party. Under the law, a jury -- or as few as 9 of 12 jurors -- can find a company as little as 1 percent responsible and then award pro-rated compensatory, but unlimited punitive, damages.

In many states, too, three federal appeals courts have ruled that the federal mandatory health warnings, which have been on cigarette packages since January 1966, bar injured smokers from claiming that the manufacturers didn't reveal additional information in their possession about smoking hazards.

The so-called preemption rulings don't figure at all in the Horton case. And they have knocked out only part of the complaint in the case set for trial on Jan. 19 in U.S. District Court in Newark. The plaintiff, the widower of Rose Cippolone, is suing Liggett Group Inc., Philip Morris Inc., and P. Lorillard Inc. Mrs. Cippolone died of lung cancer in 1984, when she was 58. But she smoked heavily for 24 years before the mandatory warnings took effect, and the preemption rulings are irrelevant to that period.

Although industry observers generally regard the Horton lawsuit as stronger than the Cippolone suit, New Jersey plaintiff's lawyer Marc Z. Edell is persevering partly because of industry documents he has turned up in pretrial discovery. In fighting pretrial disclosure of the papers, the companies themselves have spoken of "the danger ... of embarrassment, oppression and apparent incrimination" that would arise were some of them to be "taken out of context and left unexplained."

No one appears to doubt that Cippolone, not to mention Horton, are much stronger cases than the pair that went to trial two years ago after a string of courtroom defeats brought smokers' litigation to a halt in 1970.

In the better-known 1985 case, the family of John M. Galbraith, represented by Melvin M. Belli, sued R.J. Reynolds Tobacco Co. in Superior Court in Santa Barbara, Calif. As it turned out, Belli's medical evidence tying Galbraith's death to smoking was highly questionable, no autopsy was performed, and the pretrial discovery was almost nil on crucial issues, such as the purity of the product and cigarette advertising. The contrast with the Horton and Cippolone cases is stark.

Reynolds prepared with exemplary thoroughness (including hiring a psychological research firm to select juries for mock trials), and, at trial, stressed that Galbraith was personally responsible for his decision to smoke.

The consensus in Santa Barbara was that Reynolds was so confident of winning that it privately welcomed the trial, counting on a 12-to-0 victory that would chill potential litigants for years to come. Indeed, the company said last week, the number of suits filed against it fell from 60 in 1986 to 12 in 1987.

Judge Bruce W. Dodds also expected a 12-to-0 victory for Reynolds, he said in an unusual interview in Tobacco on Trial, a Boston-based newsletter, last June. Instead, Belli got three votes; if he'd gotten 4, the jury would have been hung.