Hasenclever, H. F. (U. S. Public Health Service, Bethesda, Md.), and William O. Mitchell. Antigenic studies of Candida. I. Observations of two antigenic groups in Candida albicans. J. Bacteriol. 82:570-573. 1961.-Two distinct antigenic groups, detected by tube agglutination, have been observed among strains of Candida albicans. Adsorption of an antiserum with a heterologous strain did not remove the agglutinating properties for the homologous strain. Homologous adsorption of the antiserum did remove its agglutinating properties. Seventy-one isolates of C. albicans have been screened with this adsorbed antiserum and 38 were agglutinated at a serum dilution of 1:240 to 1:480 (group A), whereas 33 were not agglutinated at a serum dilution of 1:30 (group B). Thirty-five of the strains were studied with rabbit antisera prepared against each of six strains (three from each group). Agglutination reactions of these strains with samples of each antiserum adsorbed individually with each immunizing strain verified the results of the screening agglutination reactions. All the agglutinating properties of antisera prepared against the three group B strains were removed by adsorption with suspensions of either group A or group B strains. The identity of all isolates used in this study was confirmed by chlamydospore formation, fermentation, and carbohydrate assimilation reactions.