Monocytes and macrophages synthesize thromboplastin when induced by a number of compounds. Our experiments show that cyclosporine A (CyA) enhances the synthesis of procoagulant activity in monocytes and in whole mononuclear cells (MNC) about two-fold when phytohaemagglutinin (PHA) is the stimulant. The enhancement is inhibited by actinomycin D and cycloheximide. CyA alone had no effect, but in the presence of PHA or phorbol ester CyA induces or enhances the synthesis of a vitamin K-dependent procoagulant in the MNC cultures. This procoagulant acts synergistically with thromboplastin synthesized in the same cultures.