THE LABOR DEPARTMENT has finally been forced in court to take an elementary step that should have been a first order of business 17 years ago, when the Occupational Safety and Health Act was passed. It has issued a regulation under which employers must warn employees who are exposed to dangerous substances in the course of their work.

You wonder why it took so long. Though the law did not require it, Congress contemplated an informed work force, and labor has consistently called for a warning program. To conservatives as well, warnings would seem the preferable approach. Surely they would rather have alerted workers exercising self-help than a government agency muscling through the work place in the workers' behalf.

But OSHA has always been a special case, the ultimate bureaucratic Land of No Return. Franz Kafka once worked in a precursor of OSHA -- a Workers' Accident Insurance Institute in Prague. He would have been right at home in its successor. Blithely authorized by a Congress that skipped over the difficulties involved, this effort to make the work place safe was begun in the never fully sympathetic Nixon and Ford administrations. Sour jokes quickly grew up around an agency that could busily issue specifications for safe ladders and the perfect toilet seat while everywhere around it workers were dying of chemical poisoning. The Carter administration was better disposed, but its record was sluggish too. Not until it was literally leaving town did it manage to issue a proposed regulation requiring that workers be notified of exposure to hazardous compounds. Almost the first act of the Reagan administration was to rescind that requirement.

In 1983 the Reagan administration issued a lesser regulation applying only to chemical manufacturers and other manufacturers who used their products. Labor and public-interest groups went to court, in part on grounds that the department could not legitimately distinguish between manufacturing and other workers; the chemicals were no less dangerous to a worker outside the manufacturing sector than within. In 1985, OSHA was ordered to expand the regulation to cover all workers. The administration continued to delay, was taken back to court and finally issued the required regulation this week.

The number of covered workers will rise from the 14 million to about 32 million. Estimates are that chemically related deaths and illnesses among the newly covered groups could fall by 20 percent. There's a cost to compliance as well; it has been estimated at perhaps $700 million the first year, $100 million a year thereafter. But surely that is not too much. You are asking simply that workers be told what their employers or their employers' suppliers already know -- that the stuff in the drum or tub next to them can do them harm. The pity is that it took so much time and the courts to extract the regulatio