Polyclonal anti-CN antibodies are used for the serotyping of CN clinical isolates and the diagnosis of cryptococcosis. They contribute to the characterization of CN capsular components. Many in vitro studies suggest that anti-CN antibodies, essentially IgG, could, if present, participate in yeast killing in vivo (T.R. Kozel, J.L. Follette, 1981, Infect. Immun., 31:978). However, previous attempts to protect mice against experimental cryptococcosis by passively transferred immune sera led to rather negative results. We report the production and characterization of a monoclonal mouse anti-CNPS antibody and evaluate its protective effect against experimental murine cryptococcosis.