An emotional public hearing on the alleged cancer treatment Laetrile opened today with proponents calling the hearing a "kangaroo court" and a psychiatrist testifying that the use of the substance is suicide.

About 40 persons were scheduled to testify at the hearing being held by the federal Food and Drug Administration. The Tenth U.S. Circuit Court of Appeals ordered the hearing on behalf of a cancer patient seeking to obtain the substance.

Also know as amygdalin and vitamin B-17, Laetrile occurs naturally in the pits of apricots, peaches and bitter almons. It has been promoted as a cancer "cure" for about 25 years.

"Congress has passed no law against apricot pits," said Michael L. Culbert, of Oakland, Calif., a spokesman for an organization called Committee for Freedom of Choice in Cancer Thereapy. "Although this is a kanagroo court, we want the record to show we were here."

About 150 people, most of them supporters of the substance, attended the opening session in the grand ballroom of the Radisson Muehlebach Hotel, applauding and cheering those they agreed with and jeering opponents of Laetrile.

One of those who spoke against the use of Laetrile was Dr. Samuel C. Klagsbrun, a psychiatrist at St. Luke's Hospital in New York.

"The hoax of Laetrile is being sold to you, the public, in an extremely powerful, insidious manner, because of the emotional aspect of this experience," he said. "I know about cancer and death at first hand. That's my work. I treat people who are looking for magical cures . . . It is suicide we're talking about."

The audience did not agree with Klagsbrun, and applauded such witnesses as David H. Ball of Ferguson, Mo., a 76-year-old retired chiropractor who testified he had been cured of cancer of both breasts by Laetrile treatment.

"I am a cancer victim who has been cured," Ball said.

Dr. John Jennings, associate FDA commissioner for medical affairs, conducted the hearing, which is expected to last through Tuesday.

"We're not attempting to reach a final conclusion on the safety and efficacy of Laetrile," Jennings said. The purpose of the hearing is to compile a record of the available evidence about the substance.

The FDA says it has found no evidence to support claims that Laetrile can cure or prevent cancer. Proponents claim Laetrile is not a drug but a vitamin, and should not be subject to federal drug laws.

The substance cannot be sold legally in the United States, but is permitted in 26 other countries and is smuggled in from Mexico and Canada. Alaska recently legalized its use, however, and the state legislature in Indiana last Saturday overrode a veto by Gov. Otis Bowen, a doctor, on a bill permitting use of Laetrile.