Human fibroblasts can express numerous regulatory molecules that influence immune function. IL-16, a ligand for CD4, is a chemoattractant molecule expressed by lymphocytes, eosinophils, mast cells, and lung epithelium. It appears that the sole target for IL-16 is the CD4-bearing cell. Here we demonstrate that fibroblasts from several tissues can express IL-16 mRNA and protein as well as IL-16-dependent chemoattractant activity. The transcript is expressed abundantly under basal culture conditions as a 2.5-kb band on Northern analysis, similar to that observed in lymphocytes. IL-16 protein and activity are undetectable in fibroblast cultures under these same control conditions. However, when treated with proinflammatory cytokines such as IL-1beta, they express very high levels of IL-16 protein and chemoattractant activity, a substantial component of which can be blocked with IL-16-neutralizing Abs. The amount of IL-16 protein released into the medium is 3- to 4-fold greater, on a per cell basis, than that observed in lymphocytes. The induction of IL-16 protein by IL-1beta can be attenuated with specific inhibition of caspase-3, which could be detected in IL-1beta-treated fibroblasts. IL-1beta also induces RANTES mRNA, protein, and activity, and most of the chemoattractant activity released from fibroblasts not derived from IL-16 can be attributed to RANTES. Human fibroblasts appear to be an important source of IL-16 and through expression of this molecule may have key roles in the recruitment of CD4+ cells to sites of inflammation. IL-16 expression and the mechanism involved in its regulation appear to be cell type specific.