As recently reported, a strong stimulation of noninfected CD4+ H9- cells by Sarcocystis gigantea (syn. S. ovifelis) extract (SGE) was observed using the lymphocyte proliferation assay. After SGE prestimulation, human immunodeficiency virus (HIV)-infected H9+ cells showed an exacerbation of viral replication. In the present study we investigated the reactivity of HIV-infected human monocytes using SGE. The highly sensitive p24 core-profile enzyme-linked immunosorbent assay (ELISA) was used to examine directly the amount of HIV produced. Experiments were performed using U937 permanent monocytic cells. Permanent incubation as well as preincubation with SGE before virus infection stimulated HIV expression in all the cells. In U937 cells the viral release per cell was 64 times higher on permanent stimulation with 320 microg SGE relative to controls and 9 times higher following prestimulation.