Antonin Scalia, a gregarious, piano-playing federal appeals court judge whom President Reagan plans to nominate to the Supreme Court, is a vigorous conservative whose relative youth, sharp intellect and outgoing personality made him a favorite for the post among conservative lawyers and activists.

Scalia, 50, was appointed to the U.S. Court of Appeals for the District of Columbia in 1982 and was described by those familiar with his work there as a brilliant legal scholar and effective consensus-builder who is likely to try to edge the high court farther to the right.

"There will be a more conservative jurisprudence -- tangibly but not dramatically so," if the Senate confirms Scalia's nomination, said Bruce Fein, who runs a legal consulting firm here. Scalia, he said, "is not going to be a surprise" for the administration. "He's got a well-developed, articulated philosophy."

In his opinions while on the appeals court, Scalia, who worked in both the Nixon and Ford administrations and has taught at four law schools, appeared to go out of his way to express views consistent with those of the Reagan administration, often writing lengthy concurring opinions to explain his reasoning. That practice fueled speculation in legal and political circles that he was actively campaigning for a high court nomination.

"For someone who's spent his whole personal life in the law, being nominated for the Supreme Court is the culmination of a dream," Scalia said. In announcing Scalia's nomination, Reagan said his "great personal energy, the force of his intellect, the depth of his understanding of our constitutional jurisprudence uniquely qualify him for elevation to our highest court."

If confirmed, Scalia would become the youngest member of the Supreme Court.

Scalia's opinions were often cited by conservative legal activists, and yesterday they hailed his nomination to fill the vacancy created by Justice William H. Rehnquist's elevation to chief justice to replace retiring Chief Justice Warren E. Burger.

Scalia, said Washington Legal Foundation director Paul Kamenar, is "an excellent choice who will fit in well with the Rehnquist court."

The only child of a Sicilian immigrant and the first Italian American nominated to the Supreme Court, Scalia is a graduate of Harvard Law School and was well-positioned to snare the nomination. He is, for example, 10 years younger than Judge Robert H. Bork, his conservative colleague on the appeals court here who was considered to be a top contender for the high court.

"To the extent these things are ever knowable . . . Scalia is probably more predictable on particular issues than either Bork or 7th Circuit Judge Richard A. Posner," another front-runner, said University of Chicago Law Professor Geoffrey R. Stone, a former colleague of Scalia's. "On abortion, on affirmative action, on criminal procedure and on church and state, Scalia is ore likely to take views that are consistent with the administration," Stone said.

A Roman Catholic who is the father of nine children, Scalia is personally opposed to abortion and believes that the Supreme Court's 1973 decision legalizing abortion "is not supported by the constitution," Stone said.

Scalia specialized in administrative law while teaching at the University of Chicago, Stanford, and the University of Virginia. He headed the Justice Department Office of Legal Counsel from 1974 to 1977, and served in other government posts during the Nixon administration.

He has been active with the Federalist Society, a group of young, conservative lawyers whose members fill the ranks of the Justice Department, serving as an adviser to the organization while teaching law at the University of Chicago.

Scalia, known to his friends as Nino, "is not what you think of as a typical conservative Supreme Court justice who scowls at the world," said Abner J. Mikva, a liberal colleague of Scalia's on the D.C. circuit, which is often described as second in importance only to the Supreme Court. "He's a got a good sense of humor, lots of human qualities, an uproarious laugh -- he's a good storyteller."

Scalia, Mikva said, "is the kind of person you can pop in on and say, 'What do you think about this?' He'll drop what he's doing and start to kibbitz with you."

Mikva said that once Scalia makes up his mind about a case, "He'll plant his flag and go down in flames with it if he needs to ." But Mikva added that if Scalia is "still groping for a position . . . he's much more likely to reach out for consensus than to look for the furthest extrapolation he can make of a point."

Said Judge Patricia M. Wald, another liberal on the appeals court, "He's fun to work with . . . I think he does listen, he has an open mind, he enjoys the dialogue going back and forth. Even when you're on opposing sides, the process usually is enjoyable . . . . I do have the sense he has strong feelings about particular things, but that he does come to each case willing to listen to both sides."

In his four years on the appeals court, Scalia has written about 100 opinions and is credited with writing the unsigned three-judge panel opinion this February striking down the Gramm-Rudman-Hollings budget balancing law.

Scalia keeps a word processor on the desk in his chambers and, unlike many colleagues, writes most of his own opinions, employing a flashy, distinctive style.

In his opinions, Scalia has taken an extremely narrow view of the proper role of courts in mediating disputes and of the permissible scope of government regulation. In a 1983 case in which Judge J. Skelly Wright found that the Food and Drug Administration must consider whether lethal injections in capital punishment cases met the standards for safe and effective drugs, Scalia criticized the majority position, saying it had "less to do with assuring safe and effective drugs than with preventing the states' constitutionally permissible imposition of capital punishment."

Scalia has also ruled against the press in several important cases, prompting conservative columnist William Safire of The New York Times, in a column a year ago, to describe him as "the worst enemy of free speech in America today."

In a majority opinion last September, Scalia ruled that the public has no First Amendment right to see documents filed and admittted in civil court suits until after there has been a final decision in the case. On behalf of himself and Judge George E. MacKinnon, Scalia said reporters have "no First Amendment entitlement to a document-by-document determination of the need for confdentiality" until a trial is over.

Scalia joined MacKinnon in overturning U.S. District Court Judge Oliver Gasch's 1983 ruling that The Washington Post did not libel former Mobil Corp. President William P. Tavoulareas. He joined MacKinnon's dissent from the decision of the full court to rehear the case, which is still pending.

Scalia also dissented when a six-judge majority ruled in December 1984 that a Marxist professor seeking a job at the University of Maryland could not sue syndicated columnists Rowland Evans and Robert Novak for disparaging statements. The majority said the statements were "entitled to absolute First Amendment protection as expressions of opinion. Scalia, in a dissent, said the Evans and Novak statements constituted "a classic and cooly crafted libel."

Scalia's cases on the appeals court here have not touched on some of the most contested issues that come before the high court: affirmative action, abortion, separation of church and state, and other politically-charged questions.

But those familiar with his views said Scalia shared the Reagan administration's approach on them.

In a 1979 law review article, he expressed harsh criticism of affirmative action programs for minorities and women. "I am in short opposed to racial affirmative action for reasons of both principle and practicality," Scalia said. "Sex-based affirmative action presents somewhat different constitutional issues, but it seems to me a poor idea . . . . "

Scalia acknowledged that "a person espousing the views I have expressed . . . exposes himself to charges of, at best, insensitivity or, at worst, bigotry."

However, he said, "I must content myself with the observation that it must be a queer sort of bigotry indeed since it is shared by many intelligent members of the alleged target group . . . . Some of the most vocal opposition to racial affirmative action comes from minority group members who have seen the value of their accomplishments erased by the suspicion -- no, to be frank, the reality -- of a lower standard for their group in the universities and the professions. "This new racial presumption, imposed upon those who have lifted themselves above the effects of old racial presumptions, is the most evil fruit of a fundamentally bad seed. From racist principles flow racist results."

Scalia was born March 11, 1936 in Trenton, N.J. He graduated summa cum laude and first in his class from Georgetown University in Washington and magna cum laude from Harvard Law School, where he served as an editor of the Harvard Law Review. In 1960, he married Maureen McCarthy.