The rat small intestinal epithelial cell line, IEC-6, was examined for the presence of IL-1 mRNAs using the reverse transcription/polymerase chain reaction method. IL-1 alpha but not IL-1 beta transcripts were detected in plastic adherent cells. The levels of IL-1 alpha transcripts were similar in cells cultured at different densities. IL-1 activity was not detected, by bioassay, in the culture supernatant of the cells, nor was it membrane associated. IL-1 activity was detected in cell lysates, although its measurement was made difficult by the presence of an inhibitor of the bioassay. Intracellular IL-1 alpha was detectable using Western blots. Interleukin-1 receptor antagonist mRNA was also detectable in plastic adherent cells. Treatment with TGF-beta 1, IL-1 beta, IL-6, TNF-alpha, or combinations of any two of these cytokines failed to induce the secretion of the IL-1 alpha or to significantly change the levels of mRNA. The IEC-6 cell line is similar to other epithelial cells in having intracellular pools of IL-1 alpha.