The nation's giant companies can mount hard, aggressive expansion campaigns without violating antitrust laws -- even though a monopoly may result -- provided that their actions are "reasonable," the Federal Trade Commission concluded yesterday in a major ruling on antitrust policy.

The FTC commissioners dismissed the agency's own suit against E.I. du Pont de Nemours & Co., which charged that the chemical firm had tried to monopolize the market for titanium dioxide, a chemical pigment widely used to whiten paints, paper and other products. The chemical accounted for $257 million of Du Pont's $8.3 billion sales in 1976, two years before the FTC filed its complaint.

The commissioners said they agreed with a September 1979 decision in Du Pont's favor by FTC Administrative Law Judge Miles J. Brown. He held that while Du Pont's efforts to expand its production of titanium dioxide may have discouraged competitors from meeting Du Pont's challenge, no antitrust laws were violated. Yesterday's commission ruling was on an appeal by the FTC staff of Brown's ruling.

The FTC staff had charged that Du Pont embarked on a campaign in 1972 to monopolize the titanium dioxide market by taking advantage of its unique, cheaper process for making the chemical from low-cost titanium ores.