VACAVILLE, Calif., Jan. 17 — 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.

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