Of the several morphological and physiological criteria that have been proposed to identifyTrichophyton rubrum andTrichophyton mentagrophytes differentially, only the hair invasion test agrees with the immunological results reported here. A typical strain ofT. rubrum andT. mentagrophytes was obtained from the Centraalbureau voor Schimmelcultures (Netherlands) and was used as reference strain. All isolates that did not invade hair shafts (in vitro) and that produced three precipitate lines in agargel diffusion tests against anti-T. rubrum serum (refence strain, Tr-1) were placed in one group. These isolates all also produced a red undersurface pigment and macroconidia with long parallel sides. They were labledT. rubrum. Other isolates that did invade hair shafts, and produced only two precipitate lines against anti-T. rubrum serum were placed in another group and were labeledT. mentagrophytes. These isolates had variable characteristics of pigment production, colony and conidial morphology. The antigenic studies thus corroborate the validity of the hair invasion test. The hair invasion test will probably remain in wider use because of its simplicity.