A fluorescent antibody reagent was prepared with Pythium graminicola and tested for specificity by staining Pythium and Phytophthora species on glass slides.  The intensity of fluorescence could be used to separate P. graminicola from 17 other Pythium spp. and from two Phytophthora spp.  No difference in fluorescence was noted among 15 different isolates of P. graminicola.  Evidence of specificity of staining reactions, antigenic relationships of Pythium spp., and descriptions of methods used are presented.