Cars, Cameras, computers or bananas - you can sell anything in Europe's 250-million-strong Common Market, but you had better be darned careful about how you do it, especially if you're big business.

That is the increasingly clear message being bleeped to major U.S. companies who are finding business rules difficult to follow in a Europe where bigness just isn't best. Not unless you are politically protected.

No surprise, then that American firms, already operating in a high cost, low-growth European economy, now are focusing growing concern on the Common Market's stiffening antitrust laws directed at big business. Sparked by the massive, $1 million fine slapped on United Brands by the EEC in mid-February, business tensions could increase in the coming weeks as Europe's trust busters prepare to tackle another U.S.giant.

Top EEC officials predict that Eastman Kodak, will shortly be on the receiving end of a controversial Common Market decision involving an antimonopoly complaint.

Yet while the Common Market is busy castigating unfair competitive practices by major U.S. corporations, European governments, with Britain in the forefront, are giving an irate thumbs down to attempts by the Antitrust Division of U.S. Attorney Gerneral Griffin Bell's Justice Department of pursue investigations in Europe of international cartels distorting American market conditions. A case in point is the alleged uranium price-fixing cartel, where the Justice Department's efforts to conduct a criminal inquiry in Europe constantly have been blocked by European governments invoking national sovereignty.

But no such political protection exists for U.S. companies in their tussels with the EEC's antitrust authority, the Common Market's Executive Commission.

Besides United Brands, the commission already has used antimonopoly rules to attack a series of American multinationals, including General Motors Corp., Commercial Solvents Corp., and Continental Group, and is pursuing an extensive inquiry into International Business Machines' operations in Europe. Kodak is likely to be the EEC's next "cause celebre."

European antitrust officials admit that they are seeking to force the world's No. 1 camera manufacturer to divulge confidential legal information which they need to buttress an antimonopoly case they are preparing against the American company. They object to Kodak's marketing its films and development in Europe in a single sales package, claiming that this practice forces independent film developers out of the European market.

But Kodak objects to the breach of legal privilege the EEC would commit in forcing disclosure of secret information, admit Common Market officials. However, they are planning a decision which would enforce this. Such a move, it is felt here, could further increase American business alarm at the growing discretionary powers of EEC antitrust activities. This power is not trammelled by the due legal process which must be followed in the U.S.

Without this due legal process afforded by the U.S. judiciary system and the political protection cloaking many European state-run competitors, American multinationals can find EEC antitrust rules a tough proposition. Hence the claim made recently by Joe Sims of the U.S. Antitrust Division that, "If multinationals had their choice, American antitrust principles would do quite nicely as a model for international antitrust codes."

Not that the EEC is specifically out to get U.S. multinationals, but simply big business abuses in general. The trouble is that many of the biggest firms in Europe are American. "We're not looking for U.S. companies: They're simply there and they're big," explains one EEC trust buster.

United Brands was the latest victim of mushrooming EEC antitrust power, when the European Court of Justice - the EEC's highest legal authority - recently endorsed the charge that United had "abused" its dominant position on European markets for bananas, when in fact it held less than half the markets under investigation. Reacting bitterly, the company charged in return that the decision was "of ominous significance to every competitive trading company in the EEC."

Meanwhile, the EEC's attitude to international price-fixing cartels appears ambiguous.

According to sources in this city, it is vigorously investigating a cartel of paper pulp manufacturers, including seven U.S. companies. But this enthusiasm fades rapidly when, as in the case of the uranium cartel, interests are involved which European governments wish to protect.

The truth is that when the chips are down, the EEC antitrust authority must be responsive to political pressure to an extent unknown by its American counterpart at the Department of Justice. This is vividly illustrated by a letter just sent to John Shenefield, assistant attorney general for antitrust in Washington, by Willy Schlieder, the German director general of the European antitrust authority. In it, he warns that "the problems which could arise from the application of American antitrust legislation to enterprises establisheed within the Common Market concern the sovereignty of (EEC) member states."

In other words, "Hands off, Uncle Sam."