Leukoregulin, a 50-kDa T lymphocyte-derived cytokine, influences the synthesis of collagenase, stromelysin-1, collagen, and hyaluronan in human fibroblasts and is thus a determinant of extracellular matrix economy. We studied the effect of leukoregulin on the expression of plasminogen activator inhibitor type 1 (PAI-1) in human orbital and dermal fibroblasts. The lymphokine upregulated 35S-labeled PAI-1 protein expression in orbital fibroblasts in dose-dependent manner. The effect on extracellular matrix-associated PAI-1 evolved over several hours and was maximal at 10 h, when levels were 75-fold higher than controls, and then fell by 24 h. Leukoregulin treatment increased prostaglandin E2 production in orbital cultures after 24 h. When this increase was blocked with indomethacin, peak PAI-1 levels were maintained. Northern analysis demonstrated a substantial induction of steady-state PAI-1 mRNA levels within 6 h of treatment in orbital cultures. In contrast, leukoregulin lowered PAI-1 protein levels dramatically in skin fibroblasts from the abdominal wall. With regard to PAI-1 expression, it would appear that the anatomic site of origin of fibroblasts is a crucial determinant of the cellular response to leukoregulin.