The ability of Pneumocystis carinii to induce TNF-alpha release by macrophages from adult healthy humans was investigated. Monocytes and monocytes derived macrophages produced an high amount of TNF-alpha when exposed to P. carinii cysts obtained from rats with steroid induced pneumocystosis. TNF-alpha release was P. carinii specific as shown by the inhibition exerted by the anti-P. carinii hyperimmune serum and it was not mediated by putative traces of endotoxin.