A federal appeals court panel in New York added its stamp of approval to the FBI's Abscam investigation yesterday, rejecting an immigration officer's claim that the undercover operation violated his constitutional rights.

Judge Irving R. Kaufman, writing for a unanimous panel of the 2nd U.S. Circuit Court of Appeals, upheld the conviction of Alexander A. Alexandro Jr., an Immigration and Naturalization Service employe who was the first figure caught in Abscam.

Alexandro was found guilty last fall of taking a bribe from undercover agents posing as representatives of an Arab sheik in return for promising to provide the "green card" an alien needs to become a permanent resident.

Defense attorneys and prosecutors said they consider yesterday's decision significant because it comes just a few weeks before the 2nd Circuit hears oral arguments in the Abscam convictions of four members of Congress, and because it was written by a widely respected senior judge.

The decision also comes at a time when the Senate is considering convening a special investigation of the undercover techniques used in Abscam, following the resignation of Sen. Harrison A. Williams Jr. (D-N.J.).

Convicted of bribery and conspiracy, Williams argued that the FBI had entrapped him. He fought Senate discipline until it became clear he would be expelled.

The 2nd Circuit opinion follows one by the full 3rd U.S. Circuit Court of Appeals last month. By a vote of 7 to 2, that court approved the Abscam techniques and reinstated the convictions of two Philadelphia city councilmen.

In his opinion, Kaufman said, "Modern crime-fighting methods such as videotapes and carefully devised and supervised covert investigations often are the only means of discovering breaches of the fundamental mandate of one's office."

He added that while "individuals are protected from overreaching government conduct...we find it clear in this case that the means fully vindicated the end--apprehending by lawful means one who showed a willingness to abuse the public trust."

Kaufman wrote at another point that "the government's participation in this case simply did not rise to a 'demonstrable level of outrageousness' " needed to overturn the conviction.

Thomas P. Puccio, the Justice Department's chief Abscam prosecutor, said the decision "is especially significant because it is a clear affirmation, by one of the nation's leading jurists, of the necessity of more modern and sophisticated investigative techniques in the government's fight against official corruption."

Special correspondent John Kennedy in New York contributed to this report.