Release of 92-kd type IV collagenase/gelatinase, also known as gelatinase B, by inflammatory and tumor cells is increasingly recognized and is believed to facilitate cellular migration across basement membranes. It has been implicated in the pathogenesis of many diseases, but little is known of its cellular origin(s) and function in liver. In this study we have demonstrated synthesis and release of gelatinase B by human and rat Kupffer cells in primary culture. Northern analysis of RNA extracted from Kupffer cells stimulated with phorbol ester demonstrated a 2.8 kb transcript for gelatinase B. Immunoblotting and zymography of serum-free Kupffer cell-conditioned media demonstrated extracellular release of immunoreactive enzyme and gelatinase activity, Mr 92,000 (95,000 from rat cells). The organomercurial 4-aminophenyl mercuric acetate (APMA) activated the enzyme in vitro, indicating secretion primarily as a proenzyme. Stimulation of Kupffer cells by phorbol ester markedly induced gelatinase B release, which was inhibited by cycloheximide. In contrast, cycloheximide had no effect on constitutive secretion in culture, suggesting that there is some intracellular storage. Kupffer cell-derived gelatinase B was also partially purified and characterized. After separation by gelatin sepharose and gel filtration chromatogrpahy, gelatin-degrading activities of 95, 88, 75, and 65 kd were detected, the three lower-molecular-weight species probably representing activated forms. Enzyme activity was inhibited by ethyl-enediaminetetra-acetic acid (EDTA), but not by serine- and thiol-protease inhibitors, and was restored by zinc. Activity was also inhibited by tissue inhibitor of metalloproteinase-1 (TIMP-1) and alpha-2 macroglobulin. The partially purified enzyme rapidly degraded denatured collagens (gelatin) as well as native types III, IV, and V collagens, but had no activity against casein, types I and VI collagens.