Antibody-dependent cell-mediated cytotoxicity (ADCC) carried out by fresh and precultured peripheral blood mononuclear cells (PBMC) was compared. The results indicate that the cytotoxic capability was decreased in precultured PBMC, probably due to the effect of suppressor cells. The depletion of adherent cells before preincubation eliminated inhibitory effects, suggesting that the suppressor activity resided in this cell fraction. In addition, supernatants from adherent cell cultures were also able to suppress ADCC of non-adherent PBMC. Normal ADCC values were restored by supplementing the reaction medium with cycloheximide (Cy) 1 X 10(-3) M. The enhancing effect of Cy was exerted upon whole and adherent PBMC, while the opposite effect was observed when ADCC was carried out by non-adherent cells. The fact that Cy increased ADCC activity by precultured PBMC might rule out the possibility that suppression could be ascribed to trivial factors such as cell death, overcrowding or steric hindrance. The results presented in this report support the premise that ADCC is under active immunoregulation.