Current therapy of cryptococcosis is unsatisfactory, particularly in patients with AIDS. Experimental cryptococcosis models in DBA/2 mice were used to determine whether the murine monoclonal anticryptococcal antibody (designated Et) might potentiate the chemotherapeutic effect of amphotericin B (AmB). According to the inoculum size, these mice died spontaneously from acute pneumonia (high inoculum) or from brain swelling (lower inoculum). AmB and E, together significantly improved the survival of mice in both models compared with AmB alone. The mechanisms by which E₁ might potentiate AmB activity were investigated in vitro. When cryptococci were preincubated with AmB or another polyene antibiotic, nystatin, there was an augmented binding of E₁. AmB enhanced phagocytosis by unstimulated peritoneal macrophages in the presence of E₁ or normal rabbit immunoglobulins. Normal and immune IgG deserve further study to determine under what circumstances the chemotherapeutic effect of AmB can be enhanced.