The growth-regulatory ability of eicosanoids from arachidonic acid is poorly understood. To investigate their role in cell growth, we cultured transformed murine Leydig cells (B-1 or B-1 F) in serum-free medium supplemented with various compounds modulating arachidonic acid metabolism. The addition of 5-lipoxygenase inhibitors (AA 861, NDGA and quercetin) showed a growth-stimulative effect. On the other hand, their growth was remarkably inhibited by arachidonic acid added into the culture medium. This growth-inhibiting ability of arachidonic acid was almost completely reversed by a simultaneous exposure of B-1 cells to lipoxygenase inhibitor. Exogenously added 5-HETE inhibited cell proliferation in a dose-dependent manner. These results suggest that 5-lipoxygenase-catalyzed products from arachidonic acid have the ability to suppress the proliferation of some transformed cells.