In order to begin to define themechanisms by which lipopolysaccharide (LPS) regulates IL-1 gene expression, we have examined IL-1 RNA levels, the transcription rate of the IL-1 genes, and IL-1 mRNa stabilities in P388DI/C, RAW264.7, and murine peritoneal exudate cells (PEC). These experiments showed that total cellular IL-1 RNa levels and IL-1 transcription rates were dramatically upregulated in all three cell types. In all cases, IL-1α and IL-1β cellular RNA levels and gene transription rates were regulated in parallel. However, the profiles of IL-1 gene activation during the 24 h after LPS treatment differed in these three cell types. Additionally, culture in the presence of actinomycin D (Act D) showed differential stabilities of the IL-1α and IL-1β RNAs in these cells. In peritoneal exudate cells, the half-lives (t1 2) of the IL-1α ad IL-1β RNAs were each >8 h. In RAW 264.7 cells, the stability of the IL-1β RNA was greater than the IL-1α RNA (t1 2 >8 h and ∼6 h, respectively). In P388D1/C cells, the t1 2′s of the IL-1α and β RNAs varied depending on the time of addition of actinomycin D. This and other data suggest that components of the IL-1 RNa catabolic pathway are labile and sensitive to treatment with actinomycin D. Together these data indicate that the two IL-1 genes show a diverse regulatory repertoire, even within related mononuclear phagocytic cells. © 1993 Academie Press Limited.