Murine peritoneal exudate cells (PEC), analyzed immediately after isolation, did not express detectable IL 1 activity or IL 1-specific mRNA. Stimulation of these cells by adherence induced the expression of intracellular, membrane, and extracellular IL 1 activities within 4 hr. Analysis of mRNA from these cells showed a concurrent induction of both IL 1α and IL 1β mRNA within 1 hr. However, this stimulation of IL 1 expression was transient, since PEC cultured for 5 days no longer expressed IL 1 bioactivity or specific mRNA. Stimulation of these quiescent cells with bacterial lipopolysaccharide induced the re-expression of intracellular, membrane, and extracellular IL 1 activities as well as IL 1α and IL 1β mRNA. We found no qualitative difference in the degree or rate of induction of IL 1α compared with IL 1β mRNA. These results indicate that resting macrophages are IL 1 negative, and that the IL 1 inducing stimuli used in this study act transiently to increase the levels of IL 1α and IL 1β mRNA.