The National Cancer Institute will start a long-delayed trial of Laetrile on 200 to 300 cancer patients this spring or summer if the drug passes two new safety tests in animals and human beings.

One, already started, is a study of the controversial and unapproved cancer drug in rabbits.

The other, if the drug clears the rabbit tests, will be a study at the Mayo Clinic in Rochester, Minn., of its physiologic effects, and any possible ill effects, in six healthy persons.

Dr. Arthur Upton, NCI director, said in September 1978 that his institute would sponsor a clinical trial, the first large-scale test in cancer patients, of this disputed drug.

In December 1978 NCI formally asked the Food and Drug Administration for permission to make the test. "FDA has kept asking questions and asking for more data" since, Lorraine Kershner, NCI aide, explained yesterday.

Most recently, FDA asked for the new rabbit test because of a feverish reaction in one of three rabbits previously tested. The new rabbit test will take another few weeks, then the human safety testing will take two to three months, Kershner reported.

The drug's trial in cancer patients for whom there is no other treatment -- or in whom other treatments have failed -- would then take a year. It would take place at the Mayo Clinc, Memorial Sloan-Kettering Cancer Center, New York, the University of California at Los Angeles and the University of Arizona, Tucson.

The human safety test will include a special diet with supplementary vitamins and an enzyme that the cancer patients would consume later. Laetrile backers say such a diet must be used with the drug.

The FDA bans interstate Laetrile shipments, but several states have legalized its use, and cancer patients can import it under an affidavit system created by a federal judge in Oklahoma. The FDA is fighting the affidavits in court.

Laetrile, also known as amygdalin, is made from apricot pits, usually, and contains cyanide, a poison. NCI officals have said they, too, think it is valueless, but feel only a fair, scientifically conducted trial can settle the question in many persons' minds. A test is needed, according to Dr. Charles Moertel of the Mayo Clinic, because "Laetrile has assumed proportions that no other quack medicine has assumed before."