Preculture of thioglycollate-elicited C3HeB/FeJ mouse peritoneal macrophages in vitro with subthreshold stimulatory concentrations of lipopolysaccharide (LPS) can induce hyporesponsiveness (desensitization) to both tumor necrosis factor alpha (TNF-alpha) and nitric oxide (NO) production when these cells are subsequently stimulated with 100 ng/ml of LPS. We have established, however, that the primary dose of LPS required for inducing downregulation of NO production is significantly lower than that required for inducing downregulation of TNF-alpha production. Further, when LPS-pretreated macrophages become refractory to subsequent LPS stimulation for NO production, the secondary LPS-stimulated TNF-alpha production is markedly enhanced, and vice versa. These results indicate that LPS-induced TNF-alpha and NO production by macrophages are differentially regulated, and that the observed desensitization process may not reflect a state in which macrophages are totally refractory to subsequent LPS stimulation. Rather, our data suggest that LPS-pretreated macrophages become selectively primed for differential responses to LPS. The LPS-induced selective priming effects are not restricted to LPS stimulation, but extend as well to stimuli such as zymosan, Staphylococcus aureus, and heat-killed Listeria monocytogenes.