Agglutinins have been prepared against the medically important Candida species. Crude antisera to the various species demonstrated intense cross-reactions with heterologous yeastlike fungi as well as with many true yeasts. However, carefully monitored adsorptions of selected antisera allowed the production of six factor sera that proved useful in a slide agglutination test. These six sera permitted the rapid and specific identification of C. guilliermondii, C. krusei, C. parapsilosis, and C. pseudotropicalis. They also allowed the delineation of two groups: (i) C. albicans (type A)-C. tropicalis and (ii) C. albicans (type B)-C. stellatoidea. C. albicans type A could be readily distinguished from C. tropicalis by its ability to form germ tubes in serum. C. stellatoidea could be distinguished from C. albicans type B by its predominantly filamentous growth on a nutritionally deficient medium. The medically important Candida species could be identified within 24 hr by the combined use of serological and morphological procedures.