Patients with disseminated infections caused by Trichosporon beigelii have a circulating antigen that cross-reacts with the polysaccharide capsule of Cryptococcus neoformans. We studied the localization of this antigen by immunoelectron microscopy in a rabbit model of experimental disseminated trichosporonosis. Deparaffinized lung sections were examined by using a murine monoclonal anti-cryptococcal polysaccharide antibody and colloidal gold particles coated with goat antibody to murine immunoglobulin G. Antigen that cross-reacted with the monoclonal antibody was observed in the T. beigelii cell wall and in a fibrillar matrix extending from the cell wall.