The antigenic profiles of 10 mucoraceous fungi--Absidia corymbifera, Mortierella wolfii, Mucor miehei, M. pusillus, M. racemosus, Rhizopus arrhizus, R. microsporus, R. oryzae, R. rhizopodiformis, R. stolonifer,--Candida albicans and Aspergillus fumigatus were compared by crossed immunoelectrophoresis (XIE). Antigen-rich material was obtained from homogenized hyphae (or yeasts in the case of C. albicans), and antisera by multiple subcutaneous innoculation of rabbits with macerated but viable hyphal fragments of Ab. corymbifera, M. pusillus, R. oryzae or Asp. fumigatus. Unique and common antigens were demonstrable amongst the mucoraceous species although Mort. wolfii revealed little antigenic similarity with the others. Considerable sharing of antigens between Ab. corymbifera and M. pusillus was evident. Little or no cross reactivity was seen between extracts of C. albicans and Asp. fumigatus and the mucoraeceous antisera. R. oryzae and R. arrhizus, now regarded as synonymous, revealed close antigenic similarity. On the other hand, the distinction between both M. pusillus and M. miehei--which are regarded by some as belonging to a separate genus Rhizomucor--and less thermotolerant M. racemosus was reflected in their antigenic dissimilarity. Partial separation and characterization of antigens from the crude Absidia extract was achieved by concanavalin A-Sepharose chromatography. Antigens with and without affinity for concanavalin A could be demonstrated. Cross reactivity between Absidia antigens and M. pusillus antiserum appeared to be contained predominantly in material (possibly carbohydrate) which bound to concanavalin A and could be eluted with alpha-methyl-D-mannoside.