Price-fixing, bid-rigging and carving up markets are of more concern to antitrust officials in the Reagan adminstration's Justice Department than corporate mergers, Attorney General William French Smith said yesterday.

Smith vowed "very, very vigorous" action against business tactics that threaten "our free competitive enterprise system" but said he does not consider a recent wave of mergers tor be such a threat.

Defending the administration's antitrust policy on "Meet the Press" (NBC, WRC), Smith said, "Anybody who thinks we're being slack in this area just doesn't understand our program.

"In those areas that go to the heart of antitrust activity -- price-fixing, market division or bid-rigging -- this administration is going to be very, very vigorous," Smith said. "By very, very vigorous, I mean we are going after criminal sanctions, not civil. We are going after prison sentences, not fines."

Smith did not deal directly with the administration's plan for settling the lengthy antitrust case against American Telephone & Telegraph Co. which last week brought heavy criticism from some members of Congress.

The attorney general called corporate takeovers "an important part of a healthy economic system" and said the Justice Department does not oppose mergers in and of themselves.

"Nor do we think that size makes any difference," Smith added.It is only when mergers "create anticompetitive effects that the Justice Department would step in."