A federal grand jury in Pittburgh yesterday indicted Martin Marietta Aluminum and three other producers of titanium on criminal antitrust charges for allegedly conspiring to fix prices.

The indictment charged the four companies and at least five of their employes forced up the price of titanium mill products and restrained competition in the $135-million-a-year titanium business.

The charges carry maximum penalties of a $1 million fine for each of the companies and a $100,000 fine and three years in prison for the individuals.

The Justice Department's antitrust division also filed a civil lawsuit against the four companies seeking an injunction to prevent future price fixing.

Named in the indictments were RMI Co. of Niles, Ohio, its former president J. William Price Jr., and former vice president for sales. Andrew N. Eshman; Crucible Inc. of Robinson Township, Pa., and product manager, Robert Thomas; Lawrence Aviation Industries, Port Jefferson, N.Y., and vice president Gerald Cohen; and Martin Marietta Aluminum of Bethesda and George Herman, former general manager for specialty metals.

Martin Marietta issued a statement saying the company "categorically denies that it is guilty of illegal pricing actions and it intends vigorously to contest the charges."

Martin Marietta said the indictment "apparently is based upon allegations about actions of a former employe, a salesman of titanium bar products."

The company said it had cooperated fully with Justice Department investigation which had "produced no evidence that any officer or anyone else in a policy-making position in the company authorized, was involved in, or had knowledge of the acts alleged in the indictment."

A spokesman said titanium is "a very small part" of the business of Martin Marietta Aluminum, which produces about $400 million of the $1.2 billion annual sales of Martin Marietta Corp.

A light grey metal, titanium is widely used in the aircraft and aerospace industries because it is strong tubing in nuclear power plants and chemical processing facilities, and in metal alloys.

Martin Marietta's titanium manufacturing facilities are in Torrence, Cal.

The four companies named in the Anti-trust indictment are all in the titanium mill products business - turning ingots of the pure metal into smaller products.

The indictment charged that from at least 1970 until 1976 the four companies conspired to "fix, maintain and stabilized" prices of titanium mill products, but provides no further details of the alleged conspiracy.