Cardinal Humberto S. Medeiros, whose outspoken defense of migrant farm workers and anti-abortion politicking reflected his dual commitment to social activism and conservative theology, died yesterday of cardiac arrest in a Boston hospital, one day after undergoing triple coronary bypass surgery. He was 67.

"Humility and compassion for the poor are the legacies Cardinal Medeiros leaves for all of us," said Sen. Edward M. Kennedy (D-Mass.), who called him "a remarkably holy and deeply sensitive religious leader."

Medeiros, a native of the Portuguese Azores who arrived in the United States speaking no English, rose to be the leader of the third-largest Roman Catholic community in the nation, estimated at 2 million people.

A man whose vivid social conscience often forced him to take a stand on controversial issues despite a natural humility, Medeiros led the Boston archdiocese through an era of violence and racial tension. A strong supporter of civil rights, Medeiros declared, "We will not expand our parochial schools to sabotage the integration program."

His opposition to abortion led him just before the 1980 primary elections to release a letter calling abortion "an unspeakable crime" and warning Catholics who voted for pro-abortion candidates that they would share the guilt.

In his 1982 Easter letter, Medeiros said that he hoped to "raise the consciousness of Catholics in this archdiocese to their responsibility to help form an international consensus to reject ultimately the use of all nuclear arms." That letter foreshadowed the antinuclear stance taken earlier this year by the National Conference of Catholic Bishops.

Mederios was born Oct. 6, 1915 on Sao Miguel Island in the Portuguese Azores. His family emigrated to Fall River, Mass. in 1931, and Medeiros took a job in a textile mill for 62 cents a day and learned English in his spare time.

He studied for the priesthood at Washington's Catholic University, receiving a master's degree in 1942 and his doctorate in 1952.

He rose from associate pastor to bishop in his home diocese of Fall River. In 1966 he was named bishop of Brownsville, Tex., where he immediately became involved in a farm workers' strike for a $1.25-an-hour minimum wage. That, Mederios said, had been set by the federal government as "the smallest amount under which a citizen of this country could live like a human being."

His desire to carry his ministry to the people led him to spend Christmas and Easter in Texas jails, celebrating mass for the prisoners, and he sometimes traveled with migrant workers around the country.

Appointed in 1970 by Pope Paul VI to succeed the retiring Cardinal Cushing in Boston, Medeiros set out to diminish the Boston See's staggering debt, reported at $42 million when he took over, by selling unneeded properties. He also instituted reforms at the local parish level and put all priests on salary.

He was in poor health in recent years, several times requiring hospitalization for high blood pressure or exhaustion. He also suffered from diabetes in recent years.

He was a baseball fan. When in Rome in 1978 to help elect a successor to Pope Paul VI, Medeiros approached an American reporter outside St. Peter's Basilica and asked in a whisper, "How are the Red Sox making out?" Told that the team led their division by 8 1/2 games, Medeiros muttered, "Deo Gracias."

Funeral arrangements have not been set. A spokesman for the Vatican delegation in Washington said that the Pope will not attend.