Studies were performed on heterphile antibodies originally described by Hanganutziu and Deicher and referred to as H-D antibodies. It was confirmed that these antibodies appear as a result of injections of foreign species sera. They differ from Forssman antibodies by combining with bovine erythrocytes and from Paul-Bunnell antibodies by reacting with guinea-pig kidney. it was demonstrated that H-D antibodies react in double diffusion gel precipitation tests with: (1) crude extract of bovine erythrocyte stromata; (2) purified fraction of this extract devoid of Paul-Bunnell antigen; (3) whole bovine serum and sera of several other species; and (4) thermostable ethanol-insoluble freactions of serum and organs of oxen and several other species. These various antigenic preparations gave usually reactions of complete or partial identity with each other. In several instances, two or even three precipitation lines could be detected. H-D negative human erythrocytes became coated with H-D antigen upon simple incubation with H-D-positive sera. H-D antibodies were also detected in some pathological human sera without any indication that the patients had ever received injections of foreign species sera. Such antibodies were undistinguishable from H-D antibodies engendered by injections of foreign sera.