A pending civil lawsuit in Los Angeles may help dispel some of the mystery surrounding yesterday's surprise disclosure that the Justice Department is investigating alleged smuggling of marijuana and cocaine by two associates of Billy Carter, George L. Belluomini and his son-in-law, Ronald C. Sprague.

The disclosure, made by Philp W. Tone, special counsel to the Senate special Judiciary subcommittee investigating Carter's dealings with Libya, was barren of detail. The Justice Department refused to elaborate, declining to say even where the marijuana and cocaine had originated.

In the complaint and other court papers in the Los Angeles lawsuit, however, attorney Michael S. McDaniel alleges that Belluomini, who grows cotton on several thousand acres near Bakersfield, Calif., was involved in a scheme to "launder" huge revenues from illicit drugs through a struggling furniture importing firm.

The plaintiff is John Brennand, who in 1977 launched the Thi-Cal Trading Co. to manufacture high-quality rattan furniture in Thailand and import it for sale in southern California.

The defendants named in his civil lawsuit, filed in Superior Court in April 1979, were principally Belluomini, his son, Anthony, and daughter, Georgette, and John Wyatt, who is alleged to have been the source of smuggled-drug revenues that the elder Belluomini wanted to launder.

McDaniel, whose client seeks unspecified compensatory and punitive damages, said that the three Belluominis refused to testify at a deposition hearing, invoking the constitutional protection against self-incrimination.

In his complaint, McDaniel said, he alleges that Brennand was so short of funds that he was unable to draw a salary for almost two years. At the suggestion of Joe Leggie, who had joined Brennand in conceiving the idea of the importing venture, Brennand turned for help to Belluomini.

Belluomini bankrolled the operation with $300,000, enabling Brennand to draw a small salary, and promised him a bright future in exchange for control of Thi-Cal, McDaniel said.

In the complaint, the attorney continued, he alleges that the $300,000 turned out to be money from Wyatt that Beluomini was laundering.

The suit also charges that promises to Bernnand as to his role in Thi-Cal were made falsely, to obtain his expert help while the defendants used the company as a front for laundering the proceeds of illegal activities, McDaniel said.

McDaniel said that a criminal investigation by the U.S. attorney in Los Angeles has been under way for more than two years.

George Belluomini, who had been at the Hotel Washington here, could be reached neither there nor at his home. Sprague's lawyer, Rod Williams, told reporters that neither Sprague nor Belluomini had known of the criminal investigation.