In spite of the physiologic and pathologic importance of tumor necrosis factor (TNF), the cellular factors that govern its release by macrophages (MQ,) are poorly understood, in comparison with other secretory products.We have studied the role of MQ, heterogeneity and of plasma membrane receptors in regulating TNF release in virro. Resident and various exudate murine peritoneal MQ, populations were challenged with lipopolysaccharide (LPS) or different phagocytic particles, and TNF release assayed by cytotoxicity for L-929 fibroblasts. Resident peritoneal MQ, (RPMQ,) released a small amount of TNF in response to LPS whereas thioglycollate-elicited MQ, (TPMQ,) released high levels of TNF (5000 U/3 x 105 MQ,/ml). M@ elicited by Bio-Gel polyacrylamide beads (BgPMQ,), another nonspecific inflammatory stimulus, or early in the course of intraperitoneal Bacillus Calmette-GuCrin infection, before recruited cells become immunologically activated, released tenfold less TNF after the same stimulus. By contrast,TNF release in response to various phagocytic triggers was similar ("300-600 U/3 x 105 MWml) in all MQ, populations including RPMQ,. The response by BgPMQ, to LPS was enhanced by pre-treatment in vitro with interferon-y or thioglycollate broth. With respect to phagocytic receptor triggering we found that complement receptor type 3 (CR3) ligation or latex uptake did not mediate release of significant quantities of TNF (<48 U/3 x 105 MWml) by any MQ,, whereas ligation of the Fc receptor for IgGI/IgGZb subclasses or of receptors for zymosan particles sufficed, in the absence of ingestion, to induce release of circa 500 U/3 x los MWml TNF by all MQ, tested. Our studies show that MQ, vary in respect to priming for TNF release and that heterogeneity should be related to a particular triggering stimulus. Furthermore, the capacity of some MQ, populations to release unusually high levels of TNF depends on immune or nonspecific stimuli subsequent to the process of inflammatory recruitment.