Although the actions of GH on osteoblasts have been extensively investigated, its effects on osteoclasts remain unknown. In the present study, the effects of GH on bone resorption and osteoclast differentiation were examined in vitro. Bovine GH (bGH; 1-100 ng/ml) significantly stimulated bone resorption by preexistent osteoclasts in stromal cell-containing mouse bone cell cultures, whereas it did not affect the bone-resorbing activity of isolated rabbit osteoclasts. When bGH was added to unfractionated bone cells after degeneration of preexistent osteoclasts, it concentration dependently stimulated osteoclast-like cell formation. GH also enhanced 1,25-dihydroxyvitamin D3-induced osteoclast-like cell formation. Moreover, osteoclast-like cells newly formed from unfractionated bone cells in the presence of bGH possessed the ability to form pits on dentine slices. The conditioned medium from osteoblastic MC3T3-E1 cells or MC3T3-G2/PA-6 stromal cells pretreated with bGH stimulated osteoclast-like cell formation from mouse hemopoietic blast cells supported by granulocyte-macrophage colony-stimulating factor. On the other hand, the PCR products corresponding in size to the mouse GH receptor were detected in mouse hemopoietic blast cells as well as liver. GH concentration dependently stimulated osteoclast-like cell formation from these hemopoietic blast cells in the absence of stromal cells, and these osteoclast-like cells formed pits on dentine slices in the presence of MC3T3-G2/PA-6 stromal cells. The present study indicated for the first time that GH stimulates osteoclastic bone resorption through both its direct and indirect actions on osteoclast differentiation and through its indirect activation of mature osteoclasts, possibly via stromal cells, including osteoblasts.