The interleukin-1 (IL-1) system has been suggested to be involved in the cell-to-cell cross-talk within the testis. To identify a testicular cell source of IL-1 receptor antagonist (IL-1ra), mouse Sertoli cells were isolated, purified, cultured, and examined for IL-1ra. Our investigation revealed that Sertoli cells produce large amounts of immunoreactive IL-1ra under basal culture conditions, as examined by enzyme-linked immunosorbent assays. Its expression can be induced, showing maximum concentrations after 8 h of stimulation. Lipopolysaccharide, as well as IL-1α and -β, were found to stimulate IL-1ra production in Sertoli cells. FSH is capable to induce IL-1ra production in Sertoli cells in a dose-dependent manner. Immunocytochemical staining confirmed the presence of IL-1ra in the cytoplasma of Sertoli cells. The presence of IL-1ra messenger RNA was demonstrated by RT-PCR analysis. Our results may help to better evaluate the IL-1 activity in the testis and may indicate the involvement of IL-1ra in the autocrine and paracrine regulation of testicular cell function.