Resident adherent peritoneal cells selectively released high amounts of interleukin-1 (IL-1) activity when treated with silica. The use of anti-IL-1 antisera showed that both IL-1 alpha and IL-1 beta were present in supernatants of silica-treated macrophages. In contrast, intracellular IL-1 activity was totally neutralized by anti-IL-1 alpha antibodies and was easily converted into the mature IL-1 alpha form by autolysis in cytoplasmic extracts. Anion exchange chromatography clearly separated the two IL-1 species present in supernatants of silica-stimulated macrophages. Natural IL-1 beta was further characterized by chromatofocalization; it had an apparent isoelectric point, pI, in the range 8.3-8.6. In agreement with previous findings showing that IL-1 beta was released only by apoptotic cells, we have found that silica-treated macrophages underwent apoptosis. This was demonstrated by the characteristic laddering electrophoretic pattern of DNA extracted from silica-treated cells and by the morphology of macrophage nuclei stained with the DNA-specific dye DAPI. In addition, quantification of apoptotic cells was performed by a flow cytometric analysis based on the reduction of cellular DNA content exhibited by apoptotic cells. Treatment of macrophages with silica, therefore, results in an active process that promotes the processing and liberation of IL-1 beta.