The interaction of C3b, the major cleavage product of C3, with its receptor on human granulocytes results in important biological functions, including phagocytosis, superoxide generation, and release of a variety of enzymes, including histaminase. We have determined the binding kinetics and isotherm of trypsin-generated fluid-phase dimeric C3b at both 0 degrees C and 37 degrees C using human polymorphonuclear leukocytes (PMN). At 37 degrees C the apparent number of receptors per cell was threefold greater than number at 0 degrees C (66,000 vs 21,000), and the affinity (Ka) was slightly less (3.5 X 10(7) M-1 vs 6 x 10(7) M-1). C3b dimer binding was specifically inhibited by a F(ab,)2 anti-C3b receptor antibody. C3b dimer induced histaminase release in a dose-dependent fashion at a concentration of 1 to 4 X 10(7) molecules/cell, whereas at lower concentrations a dose-dependent inhibition of opsonized zymosan-induced release was noted. These effects were independent of IgG. In contrast, C3b monomer failed to demonstrate measurable direct binding or to induce histaminase release. Histaminase release, however, did occur after incubation of PMN with affinity-linked monomer (anti-C3 F(ab')2-C3b complexes). Monovalent complexes did not affect release. Monomeric C3, C3c, and C3d were without effect; however, affinity-linked C3 and C3c did cause release.