The cajanine (longistylin A-2-carboxylic acid) is isolated and identified from extracts of Cajanus cajan L. (ECC) , which structure is similar to diethylstilbestrol. The regulation properties of the cajanine and other four extracts of Cajanus cajan L. (32-1, 35-1, 35-2, and 35-3) were tested in human osteoblast-like (HOS) TE85 cells and marrow-derived osteoclast-like cells. By using MTT assay to test the change of cell proliferation, 3H-proline incorporation to investigate the formation of collagen, and by measuring alkaline phosphatase (ALP) activity, bone formation in HOS TE85 cell was evaluated after pretreated for 48 hours. Bone marrow cells were cultured to examine the derivation of osteoclast cells (OLCs), which were stained with tartrate-resistant acid phosphatase (TRAP). The long term effect (pretreated for 18 days) on promoting mineralized bone-like tissue formation was tested by Alizarin red S staining in HOS TE85 cells. After the treatment with cajanine (1 x 10(-8) g x mL(-1)) for 48 hours, cell number increased significantly (57.7%). 3H-Proline incorporation also statistically increased (98.5%) in those cells. Significant change of ALP activity was also found (P < 0.01) in 35-1 and 35-3 treated cells (they were 66.2% and 82.4% in the concentration of 1 x 10(-8) g x mL(-1), respectively). The long term (18 days) effects of 32-1 and 35-3 on promoting mineralized bone-like tissue formation in HOS TE85 cell were obvious. There were much more red blots over the field of vision compared with that of control group. After the treatment of cajanine, derived-osteoclast cells appeared later and much less compared with control. The inhibition of cajanine was 22.8% while it was 37.9% in 32-1 treated cells in the dose of 1 x 10(-7) g x mL(-1). It is obvious that cajanine and ECCs promoted the osteoblast cells proliferation and mineralized bone-like tissue formation in HOS TE85 cells, while inhibited derivation of osteoclast cells. All of these suggested that cajanine has the estrogen-like action on osteoblast and osteoclast, which could be developed as anti-osteoporosis drugs.