President Reagan revealed yesterday that a cancerous but probably harmless growth was removed from his nose on Thursday, the second such growth to be removed in three months.

He told reporters, "A biopsy revealed there were some cancer cells, and now I have a verdict of my nose is clean."

The president gave virtually no details. The White House has continued blacking out news about the chief executive's health. Yesterday officials decided to offer no information beyond the few, ambiguous and admittedly uninformed comments of the president.

He did not say what prompted the doctor to do a biopsy and remove more tissue or on what basis he could say there was no further cancer on his nose. He did not name the type of cancer involved, although it appears likely that it was the most common and usually harmless form, basal cell epithelioma.

Reagan announced the new operation when he walked into a news briefing yesterday on the Italian ship hijacking. He began by commenting on a small, round adhesive patch on the right side of his nose. "Not wanting you to lose any sleep at night, let me explain the patch on my nose."

He continued, "The doctor has been keeping track of me since the first operation for skin cancer on my nose and felt that there was some additional work needed. So yesterday Thursday afternoon, when we came back from Chicago, I went over there in the White House to the doctor's office and he did the additional work."

The appearance in August of a mark on his nose forced the White House to disclose that Reagan had had a small spot of skin cancer removed from his nose four days earlier, July 29.

The news of the skin cancer was withheld from the public at the insistence of First Lady Nancy Reagan, White House officials said.

The cancer removed from his nose in July was diagnosed as a basal cell epithelioma, the skin cancer which 400,000 Americans get each year. The chief cause is exposure to the sun.

That procedure in July came 16 days after Reagan's major surgery to remove a three-inch colon cancer and two feet of bowel in the area near that cancer.

Based on the wording of the sketchy statements about the two procedures on the president's nose, specialists said that the operation most likely done was one in which the growth is first cut out with a spoon-shaped device, then the area is burned to kill other cells.

In this procedure, according to Philip G. Prioleau, director of dermatological surgery at Cornell University Medical Center, it is common to take a biopsy to see if any more cancer cells survive at the base of the cancerous growth.

The president did not say whether the cancer cells found Thursday were at the same site as the first growth on his nose, nor did he make clear why a biopsy was done this week.

For a diagnosis of the kind of cancer involved in Thursday's procedure, the president said only, "I'm not a lawyer and not medical either. But I did hear the term basal cell.