Anti-Cryptococcus neoformans capsular polysaccharide (CPS) antibodies were measured by ELISA in patients with AIDS related complex or AIDS without a known history of cryptococcosis and in heterosexual healthy controls. Total and IgG anti-CPS antibody activity was rarely detected in patients, with mean levels lower than in controls, whereas IgM antibody activity was similar in the 3 groups. Since both humoral and cellular immunity appear to be of great importance during cryptococcosis, the inability of AIDS patients to synthetize specific IgG antibodies could impair an alternative host defence mechanism to cellular immunity.