The effect of lymphokines and interferon-γ on human monocyte chemiluminescence was studied by measuring the phorbol myristate acetate-induced luminol-dependent chemiluminescence. When human peripheral blood monocytes were incubated with lymphokines (culture supernatants of concanavalin A-stimulated human leukocytes), their ability to generate chemiluminescence increased rapidly, reaching a maximal level at about 4 h, and then decreased on further incubation. Similar results were obtained in experiments using a commercially available human interferon-γ in place of crude lymphokines. The increasing effect of interferon-γ on monocyte chemiluminescence was dose-dependent in the range of 0.1-102 U/ml. These results show that the monocyte chemiluminescence may be clinically useful as a simple and rapid method for the determination of human interferon-γ.