Complement proteins in serum are synthesized mostly by hepatocytes and many other cell-types have also been shown to synthesize complement in various tissues. However, polymorphonuclear leukocytes (PMNs) have never been reported to secrete complement. This paper demonstrates the synthesis and secretion of C3 and factor B by murine peritoneal exudate PMNs elicited with OK432 (Streptococcus preparation). Using [35S]methionine incorporation and immunoprecipitation, C3 and factor B produced by PMN are found to be antigenically and physically identical to macrophage C3 and factor B. ELISA analysis reveals that culture supernatant of PMN--free of macrophage contamination--contains C3 antigen, and both flow cytometric analysis and immunoperoxidase staining also demonstrate the presence of intracellular C3 using special precautions to eliminate non-specific staining. The role of complement produced by PMN is currently unknown, but it is very important to take this new finding into consideration for further clarification of the roles of complement in extravascular inflammatory sites.