VACAVILLE, Calif. (AP) _ Authorities dismissed assault charges Wednesday against a state prison inmate who allegedly splashed convicted mass killer Charles Manson with paint thinner and set him ablaze.

Inmate Jan Holmstrom apparently acted in self-defense, said James Highsmith, deputy district attorney of Solano County.

Manson, 48, serving a life sentence for the 1969 slayings of actress Sharon Tate and eight others, suffered second- and third-degree burns in the Sept. 25 attack. He was returned to his maximum-security cell following treatment in the prison infirmary.

It would ''be a waste of time and effort to pursue the case any further,'' and it appeared ''unlikely that we could get a conviction'' against Holmstrom, 36, Highsmith said.

It appeared probable that Holmstrom acted in self defense, in view of Manson's prison record, in which attacks on other inmates have been reported, he said.

Prison officials said Manson was in the hobby shop of the institution, located 50 miles northeast of San Francisco, when the other inmate drenched him with paint thinner and tossed a match on him.

Manson was burned over 18 percent of his body, mainly on his face, scalp and hands, in the attack.

He had threatened Holmstrom, a member of the Hare Krishna sect, because of his religious beliefs, Holmstrom told prison officials.

On Aug. 9, 1969, members of the so-called Manson Family invaded the Los Angeles home of actress Sharon Tate, who was eight months pregnant, and stabbed her and four others to death.

The next evening Manson led his followers to the Los Feliz home of food store magnate Leno LaBianca and his wife, Rosemary, who were found butchered. He was convicted of two other murders.

Manson, who was originally sentenced to death, was given a life term after the death penalty was ruled unconstitutional by the U.S. Supreme Court.