Atopy is widely used as a discriminant in selection for employment involving exposure to allergenic substances. The validity of this has been tested in a population with a known burden of what is largely considered to be an IgE mediated disease, laboratory animal allergy. The findings suggest that atopy is insufficiently sensitive and specific for this purpose and that this is probably true for other occupational allergic diseases. The relation between different concepts of atopy--namely, atopy defined by family history, by personal history, and by skin prick tests with common allergens--has also been examined. The subpopulations identified by these criteria differed appreciably. Different concepts of atopy should not be used synonymously as they often are at present.