The present study was performed to examine whether parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP) would stimulate osteoclast-like cell formation via soluble factor(s) released from osteoblasts and, if so, to characterize the involvement of PTH/PTHrP-responsive dual signal transduction systems [cAMP-dependent protein kinase (PKA) and calcium/protein kinase C(PKC)]. Osteoblasts-conditioned medium (CM) was obtained from rat osteoblastic osteosarcoma cells (UMR-106 cells), which had been cultured in serum free medium for 24 hrs after treatment with various kinds of reagents. The CM of osteoblasts treated with either 10(-7) M human(h) PTH-(1-34) or 10(-7)M hPTHrP-(1-34) equally stimulated osteoclast-like cell formation from hemopoietic blast cells derived from mouse spleen cells, although the CM treated with 10(-8) M 1,25dihydroxyvitamin D3 failed to affect it. The CM treated with both 10(-4) M dibutyryl-cAMP and a direct PKA activator, 10(-4)M Sp-cAMPS significantly increased osteoclast-like cell formation. The CM treated with a PKC activator, 10(-7)M phorbol 12-myristate 13-acetate (PMA) and calcium ionophores (10(-7)M A23187 and 10(-7)M ionomycin) also significantly enhanced osteoclast-like cell formation. The present study first indicated that osteoblast-mediated stimulation of osteoclast-like cell formation by PTH and PTHrP, and the participation of PTH/PTHrP-responsive dual signal transduction systems of osteoblasts in the stimulation of osteoclast-like cell formation by PTH and PTHrP.