The influence of serum on phagocytosis related to the complement system was examined by means of a kinetic phagocytosis method using IgG-coated particles, isolated polymorphonuclear neutrophil leucocytes (PMNs), fresh serum, in vitro activated sera and in vivo activated sera. The previously described opsonic properties of C3b and C4b were confirmed by the enhancement of phagocytic rate by the opsonization of IgG particles with C3 and C4. An anti-opsonic effect of serum was revealed by the initial inhibition of PMN phagocytosis of IgG-coated particles in the presence of fresh serum. In vitro activated norma fresh serum and in vivo activated SLE sera mediated a prolonged or even irreversible inhibition of phagocytosis dependent on the degree of complement activation. Investigation of this anti-opsonic effect of serum, which was heat-labile, suggested that it was caused by an inhibition of the interaction between the Fc receptor and IgG mediated by the C1q component of the C1 complex.