Although macrophage activation is induced in a complex manner by signals such as interferon-gamma (IFN gamma) and bacterial lipopolysaccharide (LPS) and depends on alterations in levels of specific proteins due to differences in gene expression, the complexity of gene regulation during macrophage activation in regard to multiple signals is not fully appreciated. To probe this question, we selected four model genes encoding for tumor necrosis factor (TNF), interleukin-1 (IL-1), and the immediate early genes JE and KC. After analyses of Northern blots for specific mRNA, LPS was found to enhance levels of mRNA for IL-1, TNF, JE, and KC. IFN gamma initiated heightened mRNA levels for JE but did not alter specific mRNA for IL-1, TNF, or KC. When IFN gamma and LPS were combined, additive effects on levels of specific mRNA for JE, enhancement of mRNA for TNF, suppressed mRNA for KC, and no effect on mRNA for IL-1 were observed. When transcription of these genes was assessed by nuclear "run on" experiments, LPS increased transcription of KC and TNF but not of IL-1 or JE, implying that the increased levels of mRNA for JE and IL-1 were attributable to increased stability of mRNA. Likewise, IFN gamma did not initiate transcription of JE. When IFN gamma and LPS were given together, IFN gamma enhanced the LPS-induced transcription of TNF and KC, suggesting decreased stability of mRNA for KC. A distinct pattern of regulation for each of the four genes was thus observed. Taken together, the data suggest that gene regulation in macrophage activation represents a complex response of enhanced and suppressed transcription and mRNA stability, the precise pattern of which depends on the stimuli given to the macrophages and the gene examined.