Peripheral blood mononuclear cells (PBMN) from Schistosoma mansoni-infected patients were exposed in a calorimeter to soluble adult worm antigenic preparation (SWAP), phytohemaglutinin (PHA), 4-beta-phorbol-12,13-dibutyrate (PDB) or interferon-gamma (IFN-gamma)-enriched supernatant obtained after calorimetric experiment. The total heat released by antigen-stimulated PBMN from infected patients was significantly higher than that produced by PBMN from normal subjects stimulated with the same antigen. Antigen-induced, cell-free supernatants obtained after calorimetric experiments were able to elicit in PBMN from normal subjects a heat release pattern similar to that seen with S. mansoni-infected cells. This activating effect, however, was fully removed by pre-incubation of the supernatant with monoclonal antibodies against IFN-gamma. This discrimination between PBMN from infected patients and normal subjects was also made possible by evaluation of the cell reactivity under stimulation with either PHA or PDB. Our results demonstrate that, due to its high sensitivity, calorimetry may become an important tool for basic immunological studies.