Leukotriene B4 (LTB4) is an arachidonic acid lipoxygenase metabolite with well-characterized effects on leukocytes. LTB4 has been implicated in acute inflammatory reactions and in bone resorption. In the present study, the effect of LTB4 on osteoblastic cells was examined. LTB4 inhibited cell proliferation in normal osteoblastic rat calvaria cells in a dose-dependent manner and had biphasic effects in the human osteoblast-like osteosarcoma cell lines Saos-2 and G292. Indomethacin did not modulate the nature of these LTB4 effects in any of the cells. However, it potentiated the high LTB4 dose effects in normal cells and G292 cells. AA861, an inhibitor of lipoxygenase, did not modulate the LTB4 effects in these two cell lines. These results suggest that LTB4 is involved in the regulation of osteoblastic cell proliferation and may interact with prostaglandins to modulate these effects.