It happened almost too quickly. On Wednesday, The Post confirmed an earlier Science magazine report that the Department of Agriculture had begun running political loyalty checks on scientists serving as expert advisers. Within hours and without comment, Agriculture Secretary John R. Block ordered a stop to the checks. A spokesman would say only that published reports had brought the matter "to the front burner."

It is a relief to see Mr. Block retreating, however awkwardly, from a major indiscretion. But before the affair disappears from view, it is important to understand just what was at stake. The now-canceled program involved peer review panels, groups of expert scientists chosen to rank research proposals according to their scientific promise. A percentage of the top-ranking proposals are then funded. The panels do not consider or advise on anything touching policy.

Membership on a peer review panel is a form of professional recognition. It is also extremely demanding in time and effort. Members must rigorously exclude professional rivalries and personal feelings, even though they may know the applicants, and they know that their decisions may make or break a research career. The temptation to steal others' research ideas can be enormous.

Scientists agree to serve, and generally serve honorably, because they value the system so highly. It has proved to be a uniquely successful way to discover and support the best research. But it is a fragile system that depends on adherence to the strictest standards of professional integrity. Politics at any level--in the choice of panel members or in their deliberations--will quickly destroy it. The system has operated without political interference for 30 years, since the days when McCarthyism prompted loyalty checks of a different kind.