We studied the mechanisms that govern the expression of interstitial collagenase and 92-kDa gelatinase in U937 cells, a human monocyte-like cell line, exposed to bacterial lipopolysaccharide (LPS), a potent inducer of metalloproteinase expression. U937 cells were differentiated by phorbol ester (phorbol 12-myristate 13-acetate (PMA)) and, 24 h later, were exposed to LPS for an additional 24 h. Enzyme-linked immunosorbent assay and Northern hybridization showed that PMA mediated an induction of collagenase and markedly stimulated the low basal levels of 92-kDa gelatinase. Subsequent exposure to LPS substantially increased the production of both enzymes. Nuclear runoff assay demonstrated that PMA regulated collagenase and 92-kDa gelatinase transcription. LPS also stimulated collagenase transcription but did not affect transcription of 92-kDa gelatinase. Consistent with the runoff data, the decay rate of collagenase mRNA did not differ between experimental treatments, but the half-life of gelatinase mRNA increased with exposure to LPS. Furthermore, in situ hybridization showed that 92-kDa gelatinase was expressed by all cells whereas collagenase was produced by a subpopulation of cells in both PMA- and PMA/LPS-exposed cultures, and similar findings were seen with LPS-activated human alveolar macrophages. These data indicate that divergent mechanisms control metalloproteinase expression in phagocytic cells and that enzyme production differs among macrophage subpopulations.