Identification of contaminating tissue fragments as "floaters" is a common problem in surgical pathology. They may be introduced at several steps in the processing of specimens, and they are capable of causing interpretative consternation. Using immunohistochemistry with monoclonal antibodies to blood group isoantigens A, B, and H, we studied nine cases in which floaters were present. In five of the study cases, discordant blood group immunostains allowed identification of the artifactual tissue fragments. By chance, the other four specimens contained probable floaters from patients whose blood groups were concordant with those from whom the "native" tissue had been obtained. These results indicate that in some instances, blood group immunostains may resolve interpretative difficulties surrounding floaters. It is likely that application of immunostains, directed at additional blood group antigens, will extend the utility of this procedure.