A major grand jury investigation here will explore the possibility that at least three national pharmaceutical concerns made illegal oral agreements to lower prices on certain household drug products to obtain larger shares of the market, sources said this week.

The Justice Department's Antitrust Division will head the investigation, which is scheduled to begin June 20. The probe was termed "major multi-district litigation" by one source, which means possible indictments would not necessarily be confined to the Eastern District of Virginia.

One company, the Richmond-based A. H. Robins Company, has already been subpoenaded. Robins, which last year had $191 million in worldwide pharmaceutical sales, has been asked to provide documentation related to marketing.

One federal source said an assumption that Robins was one of the companies under investigation would be "totally wrong," indicating the firm might instead be a victim.

Robins vice president G. E. Stiles said 85 to 90 per cent of his firm's domestic business involves sales to wholesalers. He added that he was aware of "no conspiracy."

Attorney General Griffin Bell alluded to anticompetitive pricing in an appearance Wednesday before a Senate antitrust subcommittee. "We will seek to remedy the situation either by changing the structure of the industry or by closing down the avenues of communications," he pledged.