The effects of bee pollen extract on osteoclastic bone resorption in vitro were investigated. The water-solubilized extracts were obtained from the bee pollen of Cistus ladaniferus. Femoral-diaphyseal or -metaphyseal tissues of rats were cultured for 48 hr in medium containing either vehicle, bone-resorbing factor, or bone-resorbing factor plus bee pollen extracts (10, 100, or 1000 μg/ml of medium). Culture with parathyroid hormone [human (1-34) PTH; 10^-7 M], prostaglandin E₂ (PGE₂; 10^-5 M), or 1,25-dihydroxyvitamin D₃ (10^-6 M) caused a significant decrease in calcium content in the diaphyseal or metaphyseal tissues. These decreases were completely prevented in the presence of bee pollen extracts (10, 100, or 1000 μg/ml). The presence of PTH (10^-7 M) caused a significant increase in medium glucose consumption and lactic acid production in the diaphyseal or metaphyseal tissues. These increases were significantly inhibited by culture with bee pollen extracts (10, 100, or 1000 μg/ml). Mouse bone marrow cells were cultured for 7 days in the presence of bone-resorbing factor in vitro. Culture with PTH (10^-7 M), PGE₂ (10^-5 M), tumor necrosis factor-α(10 ng/ml of medium), or lypopolysaccharide (10 μg/ml of medium) caused a significant increase in osteoclast-like cell formation. These increases were significantly inhibited in the presence of bee pollen extracts (10, 50, or 100 μg/ml of medium). This study demonstrates that bee pollen extract has inhibitory effects on osteoclastic bone resorption in vitro.