WASHINGTON (AP) _ A new type of laboratory mouse broke legal ground Tuesday as the federal government granted Harvard University the first U.S. patent on an animal.

''A few minutes ago we issued a patent which, for the first time in the history of the U.S. Patent and Trademark Office, covers an animal,'' Patent Commissioner Donald J. Quigg announced shortly after noon.

''This particular animal (is) a mouse which has been genetically engineered so as to make it more susceptible to'' cancer, Quigg said.

The patent, No. 4,736,866, named Philip Leder of Chestnut Hill, Mass., and Timothy A. Stewart of San Francisco as inventors of the mouse, which is to be used for research into cancer.

Patents on forms of life are not new, with both plants and types of bacteria having been patented in the past.

Almost any type of animal - except humans - could be considered for a patent, Quigg said.

The Patent Office said last year that it was considering patents on animals, following an appellate court ruling in a case involving a new type of oyster.

That ruling said that the agency could go beyond the plants and bacteria, to which it had previously limited patents on life forms. The oyster patent was never issued, however, with examiners concluding that the process was not original enough.

The only other animal case occurred in 1974, when the Patent Office refused to patent a dwarf chicken.

The question of patenting animals has raised some controversy in recent years, however, with opponents questioning the ethics of changing animals.

Bills have been introduced in Congress to place a moratorium on such patents until the ethical problems can be worked out, and Quigg said that Congress and the scientific laboratory are the proper forum for such debates.

The patent application for the mouse, which had been pending since June 22, 1984, met the legal requirements for a patent, and so one was issued, he said.

There are 21 more animal patent applications pending for the agency, Quigg added. Federal law requires that the subject of applications be kept secret until a patent is issued, he said.

However, Quigg said further animal patents are not expected to issue for ''some months.''

According to the mouse patent, a cancer-susceptible gene is introduced to the mouse embryo when it is first formed, at the stage of one or two cells.

The genetic material used to change the mice is spliced together from an activated version of the mouse cancer gene and a virus that causes mammary tumors in mice, explained Harvard spokeswoman Lillian Blacker.

Having mice that are more sensitive to cancer can be a major benefit for researchers, Quigg said.

Currently, mice are often exposed to massive doses of chemicals to see if cancer results. The altered mice could be exposed to chemical levels more like those encountered by humans, perhaps making the experiments more easily related to human dangers, Quigg said.

The mice have been used in research since 1984 at Harvard, Blacker said, with the introduced traits remaining true through several generations.

The sensitive mice can be used both to test the causes of cancer and cancer-fighting agents, Blacker and Quigg both said.

The Patent Office announcement drew prompt reaction on both sides of the issue.

Jeremy Rifkin of the Foundation on Economic Trends termed the action an ''arrogant abuse of power'' by the Patent Office. Rifkin, who has opposed genetic engineering, said a coalition of groups will seek to get Congress to overturn the action.

Rifkin also raised the possibility that an engineered mouse could escape and breed with wild mice, perhaps posing a danger to the environment.

But the Industrial Biotechnology Association hailed the action for providing researchers a new tool for cancer research.

Legally, a patent holder has the right to exclusive use of an invention for 17 years. In practice, most patent holders license their developments to others, collecting a fee. The idea is to encourage invention by protecting the rights of individuals who produce something new.