WASHINGTON (AP) _ Federal regulators say a $50,000 fine imposed on a California ''dial-a- porn'' company should demonstrate to other such operators that they must keep their material inaccessible to minors.

Audio Enterprises Inc., of Mill Valley, Calif., and operator Wendy King agreed to the fine and to stop using interstate telephone lines to transmit obscene messages.

It was one of the first dial-a-porn fines imposed by the Federal Communications Commission.

''The $50,000 payment that the dial-a-porn operator will have to pay to the government ... shows that there will be a high price attached to failure to obey the law scrupulously in this important area,'' Gerald Brock, head of the FCC's common carrier bureau, said Monday. ''This sends a signal there are some teeth in the law.''

FCC officials said Audio Enterprises is effectively out of business. There was no current listing for the company in telephone information.

To resume operations, King or Audio Enterprises must tone down the messages and use access codes, credit cards or scrambling equipment to prevent children under 18 from dialing. Such operations will be monitored by the FCC, according to the agreement.

The FCC launched its investigation of Audio Enterprises after a mother in California wrote that her 13-year-old son and his friends spent $74 dialing a pay-to-listen 900-number for 211 minutes.

Also listening was the woman's daughter, who was later molested by two boys who had heard the tapes, the woman wrote. ''This phone call has damaged our lives,'' the woman wrote.

The FCC informed Audio Enterprises last December that it was in apparent violation of FCC's rules that require companies to restrict access to obscene material. The commission also informed Intercambio Inc., of San Jose, Calif., that it was in violation of the law. That case is pending.

The FCC declared that the material broadcast by the two companies was obscene. Under a Supreme Court's test, material is obscene if it depicts sexual acts in a patently offensive way, appeals to the prurient interest of an average person and lacks serious literary or artistic value.

FCC officials estimated Audio Enterprises collected about $250,000 in revenues over a two-year period.

Altogether, the dial-a-porn industry produces about $54 million annually in revenue for providers and phone companies, according to the Information Industry Bulletin.

The FCC has referred some complaints to the Justice Department. In May 1987, two dial-a-porn providers pleaded guilty to failing to comply with the commission's rule. The court fined them each $50,000.

Earlier this year, Congress passed a sweeping dial-a-porn law that bans obscene and indecent commercial messages. The law has not been enforced because of legal challenges in New York and California.