Expression of plasminogen activator inhibitor type 1 (PAI-1), an important determinant of the pericellular proteolytic environment, was assessed in cultured human orbital and dermal fibroblasts. Bidimensional electrophoretic separation of total cellular [35S]methionine-labeled proteins revealed multiple isoforms of the 50-kDa PAI-1 protein (isoelectric point range 5.6-6.3) expressed in dermal strains but not in orbital cultures. The addition of human recombinant interferon-gamma (100 U/ml) to the media for 48 h resulted in a marked induction of PAI-1 in the orbital cultures (greater than 50-fold above baseline), whereas the expression in dermal cultures was either attenuated or induced modestly (5-fold). The identity of the 50-kDa PAI-1 was verified by immunoprecipitation of secreted proteins. Indirect immunofluorescence localized PAI-1 to aggregates at the ventral undersurface of the monolayer. Interferon-gamma induced several other proteins in cultured cells. The observation that untreated orbital fibroblasts do not express detectable cell-associated PAI-1 suggests that the pericellular proteolytic environment of these cells is regulated ordinarily by other factors. The economy of the extracellular matrix of orbital connective tissue may be particularly susceptible to the influence of inflammatory cytokines such as interferon-gamma.