T he CD36 leukocyte differentiation antigen, recognized by MAbs OKM5 and OKM8 and found on human monocytes and endothelial cells, has been implicated as a sequestration receptor for erythrocytes infected with the human malaria parasite Plasmodium falciparum (IRBC). CD36 is also expressed on platelets and appears to be identical to platelet glycoprotein IV. We investigated receptor activation of monocytes and platelets by anti-CD36 MAbs and by IRBC. Incubation of human monocytes with anti-CD36 MAbs or IRBC resulted in stimulation of the respiratory burst as measured by reduction of nitroblue tetrazolium and generation of chemiluminescence. Incubation of human platelets with anti-CD36 MAbs resulted in platelet activation as measured by aggregation or ATP secretion. Activation of monocytes and platelets required appropriate intracellular transmembrane signaling and was inhibited by calcium antagonists or by specific inhibitors of protein kinase C or guanine nucleotide binding proteins. Soluble CD36 inhibited binding of IRBC to both monocytes and platelets, suggesting that these interactions are mediated by the CD36 receptor. Using a cytochemical electron microscopic technique, the presence of reactive oxygen intermediates was identified at the interface between human monocytes and IRBC. These data provide support for the hypothesis that reactive oxygen intermediates produced by monocytes when IRBC ligands interact with cell surface receptors may play a role in the pathophysiology of falciparum malaria.