Marvella Bayh, 46, died of cancer yesterday at the National Institutes of Health in Bethesda. Long before her death, she had turned her battle against the disease into an example of common sense, hope and courage for millions of others.

Her death was announced by a spokesman for her husband, Sen. Birch Bayh (D-Ind.). He said Sen. Bayh was at her side when she died.

Mrs. Bayh first was afflicted with cancer in 1971 and underwent a mastectomy. She became a special representative of the American Cancer Society and gave more than 175 speeches and countless interviews on how the disease can be prevented and the importance of early detection and treatment.

In February 1978, Mrs. Bayh learned that her cancer had returned and that it would lead to her death.

She refused to abandon her work or her opptimism. She gave her last speech at Charlottesville on March 2, calling for a "full-scale war against this disease that will claim the lives of 55 million Americans who are living today . . . We now are saving one in three cancer patients, but we have the medical knowledge to save one in two."

Mrs. Bayh said in an interview last fall that the years in which she contended with cancer had been "the greatest growth period in my life. These years since cancer came to me have been the most rewarding, the most fulfilling, the happiest in my life. I have learned to value life, to cherish it, to put my priorities in order-and to begin my long-postponed dream of being useful in my own right."

In the same interview, she said , "I believe in the great power of hope and positive thinking and I believe especially in the power of God. And of course I'm praying for a miracle."

Before her first illness in 1971, Mrs. Bayh had devoted herself to her husband's political career. In his first campaign for the Senate in 1962, she gave two or three speeches a day in his behalf and made 13 speeches in the three days just before the polls opened.

In 19718 when Sen. Bayh announced he was seeking the Democratic nomination for the presidency, he said, "My wife has a better political head than anybody who works for me. Marvella is my political confidante and my adviser. I can't conceive of anyone being more valuable in my public and in my private life."

Sen. Bayh withdrew his bid for nomination for the presidency after his wife became ill.

Mrs. Bayh herself put politics behind her to concentrate on her crusade against cancer. She played no part in her husband's 1976 try for the Democratic presidential nomination.

"Politics does not play a part in my life," she said last fall. "Oh, I sometimes think we wives may have thought we made more difference than we did. Politics remains demanding in our family life-but I have learned to do what I want to do."

In addition to speaking out on cancer - the importance of regular examinations, particularly breast examinations for women, the importance of not smoking, the importance of facing the truth and of seeking early help, the importance of being hopeful - Mrs. Bayh has devoted herself to women's rights and to the role of womein in American history.

In 1973, Mrs. Bayh and her husband were cochairmen of the American Cancer Society's campaign in Indiana. A year later, she was cochairman of the society's national crusade.

In 1977, she received the James Ewing Award of the American Society of Surgical Oncologists for her work in disseminating information about the disease. Less than a month ago, she received the Hubert H. Humphrey Inspirational Award from the D. C. chapter of the American Cancer Society. The accompanying citation said she was being honored as "the woman whose spirit and courage in her personal fight against cancer are a lasting inspiration to cancer patients and to all Americans."

Mrs. Bayh was born in Enid, Okla. Her parents were Delbert and Bernett Hern. Her father was a farmer who was active in Democratic politics.

Marvella Hern became the first girl to be elected president of the Enid High School student body. She was elected governor of the Oklahoma Girls State and president of the Girls Nation in Washington. She was greeted by President Truman here and received a citation from him.

She attended Oklahoma State University in Stillwater, Okla. A skillful public speaker, she won a statewide extemporaneous speaking contest sponsored by the American Farm Bureau.

In December 1951, she won the national speaking contest in Chicago, becoming the first woman to do so. One of those whom she defeated was Birch Bayh.

The two were married in August 1952. They lived on the Bayh family farm near Terre Haute, Ind., and Mrs. Bayh attended Indiana State University. In 1960, she graduated from Indiana University in Bloomington with a degree in education. Two years later, her husband won his first term in the Senate.

Mrs. Bayh continued to gain honors in her own right. In 1964, she was namely "Indiana Woman of the Year" by the Indianapolis chapter of Theta Sigma Xi. A year later, she was chosed one of the "Outstanding Young Women of the Nation." She received the "Pride of the Plainsmen Award" from her hometown in 1967.

But her life was not without tragedy even before the onset of cancer. Her mother suffered poor health. After she died, Mrs. Bayh's father became an alcoholic. He later killed his second wife and then committed suicide. While in college, Mrs. Bayh was in a car accident that caused double vision for some months. She and her husband were in a 1964 plane crash with Sen. Edward M. Kennedy (D-Mass.) in which two persons were killed.

Last fall, Mrs. Bayh told an Interviewer about her reactions on receiving the news that she had cancer again, and that it would take her life.

"When they told me there was treatment but no cure at this time, I dropped to my knees," she said. "Two things from out of my past, when I went to church as a child, came back to me. Number one, 'where can I go but to the Lord?' and number two, 'I am weak but He is strong."

"The third feeling I had was, 'when life comes down to basics, really how little control we all have over our own lives.' And it also came to me how, even if we live to be 100, how really short life here is. And therefore, it's important to enjoy it and not rush so fast and take time to smell the roses . . ."

In addition to her husband, Mrs. Bayh is survived by a son, Evan Bayh, a student at the University of Virginia law school.

The family suggests that expressions of sympathy be in the form of contributions to the Marvella Bayh Memorial Scholarship Fund, Indiana University Foundation, P.O. Box 500, Bloomington, Ind. 47402. CAPTION: Picture, MARVELLA BAYH