Louis M. Lyons, 84, a highly esteemed American journalist and a major influence in shaping journalistic standards and abilities as head for 25 years of the prestigious Nieman fellowship program at Harvard University, died of cancer yesterday in Cambridge, Mass.

In addition to heading the fellowship program--which brings journalists chosen for promise and performance from across the nation to Harvard for a year of study--Mr. Lyons won numerous honors as a reporter for The Boston Globe and as a radio and television news broadcaster for a Boston educational station.

When he retired in 1964 as curator of the Nieman fellowship program, Harvard gave him an honorary degree and described him as the "conscience of his profession."

To the nearly 400 journalists who took part in the fellowship program during Mr. Lyons' tenure, he provided good advice and his own example. They found him modest, blunt-spoken, clear-sighted and taciturn in the tradition of his New England background and upbringing.

"He was a father to us all," said Howard Simons, managing editor of The Washington Post and a 1959 Nieman fellow.

Mr. Lyons also was known of a man of almost complete imperturbability who could nevertheless be aroused to fury by threats to journalists' efforts to seek out and publish the truth.

"He was a Gibraltar of courage and integrity . . . especially during the difficult McCarthy era," said James C. Thomson, the present curator of the fellowship program. "He kept pressing for the press to be more courageous as well as responsible."

In a speech during the McCarthy period, a time when civil liberties appeared to be in jeopardy, Mr. Lyons called on editors to speak out in defense of the rights of Americans, asserting that the journalist "has a special privilege enshrined in our Constitution because his freedom is so essential to all our freedoms.

"No newspaper editor has a right to be neutral on the basic issue of the freedoms of the mind on which America was founded."

Mr. Lyons was born in Dorchester, Mass., Sept. 1, 1897, grew up on a poultry farm in Plymouth County, Mass., and graduated in 1918 from the Massachusetts Agricultural College, now the University of Massachusetts.

After Army service during World War I, he joined The Globe, then went to work for The Springfield Republican, where his assignments included being sent in 1923 to Plymouth, Vt., to carry the news to then-Vice President Calvin Coolidge of the illness of President Warren Harding.

When Harding died shortly afterward, and Coolidge took the oath of office from his father by lamplight, Mr. Lyons was there as a reporter and witness to history.

Subsequently, he rejoined The Globe and was selected in 1938 as one of the first Nieman fellows. He became curator of the program the following year while continuing to work for the newspaper. During World War II, he reported for The Globe from Europe.

In 1951, Mr. Lyons, known as "Louie" to his friends, began his broadcast career on WGBH, which won him the Peabody and the duPont awards. Among Mr. Lyons' attributes cited by the duPont foundation were his "crystalline perception and enviable good sense."

Survivors include his wife, Catherine, of Cambridge, and a son, Richard, who was a longtime congressional reporter for The Washington Post.