Actor Pernell Roberts, who starred in two hit television series and took an active role in the Civil Rights Movement during the 1960s, died at his Malibu, Calif., home on Sunday. He was 81.

Roberts is perhaps best known as Adam Cartwright, the eldest son of rancher Ben Cartwright (Lorne Green) on TV’s “Bonanza,” also starred in the medical drama “Trapper John, M.D.,” a spinoff role from the hit show “M*A*S*H.”

Roberts, who was born in Waycross, Ga., became an outspoken advocate for civil rights and even walked alongside the Rev. Martin Luther King Jr. in the historic 1965 march from Selma to Montgomery in Alabama, according to a statement from his family. He spent much of his adult life "pursuing his dream of equality for all," the statement said.

On the set of 'Bonanza,' he protested the use of all White crews and guest stars, finding some support but never enough to satisfy his sense of outrage," the statement said.

Roberts left the popular western after the end of the sixth season in 1965, although "Bonanza" stayed in production for another eight years. While the show's storyline kept open the possibility of his return to the Ponderosa ranch, Roberts never did.

For the first decade of his acting career, Roberts performed on stage – including Broadway – before moving to Hollywood to act in movies. His first film was "Desire Under the Elms" (1958), starring Sophia Loren and Burl Ives. A year later, he landed a role in the new series "Bonanza," playing the more serious, wiser of his half-brothers, Hoss (Dan Blocker) and Little Joe (Michael Landon).

 The move to a television series "was a difficult transition," the family statement said.

 "Accustomed to freely moving from part to part, now being in a wildly popular show, without costume changes, about three grown men living at home with their father, he found did not hold much creative interest for him," it said.

 His role of Dr. "Trapper" John McIntyre – as a balding physician in a San Francisco hospital nearly three decades after returning from the Korean War – began in 1979 and lasted six years on CBS.

 He also guest starred in more than 60 TV series.

 Roberts was married four times. His only son, Chris, was killed in a motorcycle crash in 1989.

 His fourth wife, Eleanor Criswill, was by his side when he died Sunday, the family said.