The clinical symptoms of human malaria are mediated, at least in part, by the release of tumor necrosis factor alpha (TNF) by monocytes and macrophages. We have found that lysates of Plasmodium falciparuminfected erythrocytes stimulate the secretion of TNF from human mononuclear cells, and we have generated several immunoglobulin M monoclonal antibodies (MAbs) that inhibit the induction of TNF by such lysates. Here we describe the properties of MAb 5AB3-11, which causes dose-dependent inhibition of the TNF-inducing factors derived from P. fakiparum-infected erythrocytes, with a 50%o reduction in TNF secretion at nanomolar concentrations (1 to 2 ,ug/ml). The inhibitory effect appears to be malaria specific in that the induction of TNF