We determined the effect of basic fibroblast growth factor (bFGF) on osteoclast-like cell (OCL) formation in bone marrow cultures using C57BL/6 mice. Cells were cultured for 7 days with or without bFGF at various concentrations or 10⁻⁸ mol/L 1,25(OH)₂ vitamin D₃ [1,25(OH)₂D₃]. bFGF dose-dependently increased OCL formation per well (10⁻¹⁰ mol/L = 40 ± 2; 10⁻⁹ mol/L = 146 ± 13; 10⁻⁸ mol/L = 156 ± 12) compared with control (<7 per well). The effects of bFGF at 10⁻⁹ and 10⁻⁸ mol/L were similar to that of 10⁻⁸ mol/L 1,25(OH)₂D₃ (154 ± 11 per well). OCLs formed by bFGF were multinuclear, tartrate-resistant acid phosphatase (TRAP)-positive, expressed calcitonin receptors, and formed characteristic resorption pits. We also determined whether bFGF enhanced OCL formation during the early proliferative or late differentiating phases of the cultures. When bFGF (10⁻⁸ mol/L) was added only on days 1–2 or days 3–4 of 6 day cultures, there was a significant increase in OCL formation. In contrast, when bFGF was added only on days 5–6 few OCLs formed. Addition of bFGF at days 1–6 or days 1–2 and days 5–6 caused similar increases in OCL formation, which were greater than OCL formation induced by treatment for days 1–2 or days 1–4. We examined the production of prostaglandin E₂ (PGE₂) in the cultures because bFGF is a potent stimulator of PGE₂ synthesis in bone, and PGE₂ stimulates OCL formation. bFGF treatment significantly increased PGE₂ levels in 7 day cultures (controls = 1.4 ± 0.1 nmol/L, 10⁻⁸ mol/L bFGF = 132.5 ± 0.7 nmol/L). In addition, treatment of marrow cultures with the prostaglandin synthesis inhibitors, indomethacin or NS-398 (both at 10⁻⁶ mol/L), completely blocked bFGF-induced OCL formation. We conclude that bFGF stimulates OCL formation in C57BL/6 bone marrow cultures by mechanisms that require prostaglandin synthesis. This pathway is likely to be one mechanism by which bFGF stimulates resorption.