A leading Hispanic appointee in the Carter administration, under investigation for allegedly having taken a bribe six years ago, held an extraordinary news conference yesterday and all but dared the president to fire him.

Herman Sillas Jr., the U.S. attorney for Sacramento and the administration's choice last fall to become head of the Immigration and Naturalization Service, told reporters at the National Press Club that he was refusing a White House ultimatum to take a lie detector test administered by a CIA expert to clear up the bribe controversy.

The Washington Post reported in January that the Justice Department recommended Sillas be fired after he flunked two lie detector tests about a charge that he took a $7,500 bribe from a California car dealer in 1974 just before he became the state's director of motor vehicles.

Several Hispanic groups rallied to Sillas' side and made it clear his dismissal could have political repercussions for President Carter in the important Hispanic voting bloc. The six-month delay in the White House decision on the firing recommendation led some Justice officials to think the administration was looking for a way to keep Sillas in his job to avoid offending the Hispanic community.

White House counsel Lloyd N. Cutler denied that, and at his press conference yesterday Sillas said the delay was to allow his attorney to conduct his own inquiry.

Several representatives of Hispanic groups were at the press conference, and Vilma Martinez, head of the Mexican American Legal Defense and Educational Fund, issued a statement saying that Sillas had demonstrated his innocence by fully disclosing the facts.

In his meeting with reporters, which he cleared neither with the White House nor Justice Department superiors, Sillas released a detailed chronology of events in the case, including meetings with Attorney General Benjamin R. Civiletti and Cutler, and copies of correspendence with Cutler.

Cutler said in a brief phone interview yesterday that he could not comment on the unusual public airing of the matter because he still had to make a recommendation to the president.

In his late letter to Sillas' attorney, dated July 3, Cutler said he wanted the prosecutor to submit to another lie detector test by a government examiner. If he didn't agree, "we will reach our decision in this matter in the light of Mr. Sillas' refusal to take such a further examination," he said.

A statement issued by the White House press office yesterday said Cutler's primary concerns in his review have been to "protect the integrity and public reputation of the office of the U.S. attorney while at the same time affording Mr. Sillas and his legal counsel of a fair opportunity to present his side of the matter."

In a June 12 letter to Sillas' attorney, Warren L. Ettinger of Los Angeles, Cutler expressed consternation that Sillas had taken a lie detector test administered by his own hired expert without telling the government.

In the press conference yesterday, Sillas released papers saying he passed his own private test with "flying colors" and criticizing the procedures used by the FBI polygraph expert who administered the two tests he failed.

He thus was criticizing the lie detector test that is an important investigative tool of the department for which he works.

The use of the polygraph is becoming an increasingly controversial issue because of its use against Justice employes in another internal investigation of leaks about the FBI's undercover Abscam investigation. Federal courts have refused to allow the introduction of the polygraph test results in criminal trials because of its unreliability.

Sillas said yesterday that he was going public with his version of the case now because of leaks. He said he merely wanted some say on who administered a fourth test. "What he or she finds could well have an impact on the rest of my life," he said.