We have investigated the effects of human LPS-binding protein (LBP) and human bactericidal/permeability-increasing protein (BPI) on LPS-dependent activation of mouse thioglycolate-elicited peritoneal macrophages in vitro, in comparison with human PBMCs. Confirming earlier published studies, BPI inhibited, and LBP enhanced, the ability of LPS to stimulate PBMC production of the cytokines TNF-alpha and IL-6. In marked contrast to these results, under identical conditions of in vitro culture, both LBP and BPI suppressed, in a dose-dependent manner, the ability of LPS to stimulate cytokine production in mouse macrophages. Further, while human BPI also suppressed LPS-dependent NO secretion in mouse macrophages, human LBP had no inhibitory effect on NO secretion under conditions that inhibited TNF-alpha secretion. These data provide the first direct evidence that mouse macrophages may utilize two independent pathways in response to LPS, thus leading to different phenotypic responses.