Human growth hormone with 22, 000 Dal (22K-hGH) stimulates proliferation and differentiation of osteoblasts as well as production of interleukin-6 in vitro and bone formation and remodeling in vivo. To investigate whether hGH isoform with 20Kd (20K-hGH), which accounts for 10% of circulating hGH, elicits similar metabolic effects on skeletal tissues, we studied the biological effects of 20K-hGH in cultured human osteoblast-like cells (HOB). HOB were obtained from trabecular bone explants and cultured in α-MEM supplemented with 10%FCS. In subconfluent cultures, 22K- and 20K-hGH stimulated [³H]thymidine incorporation by 62±27% and 63±23%, respectively (mean±SD, n=8, P>0.1). In confluent cultures, 22K- and 20K-hGH increased alkaline phosphatase activity by 38±23% and 41±23% (P>0.1), respectively, and increased the osteocalcin concentration in the presence of 10^-9M 1, 25-(OH)₂D₃ by 50% and 47% (P>0.1), respectively. Furthermore, both hGHs doubled the interleukin-6 (IL-6) concentration in the conditioned medium. RT-PCR analysis revealed that 22K- and 20K- hGH increased IL-6 gene expression 2.2±0.6 and 2.4±0.7 -fold, respectively. In summary, we have demonstrated that 20K-hGH elicits equipotent anabolic effects on HOB and stimulates to the same extent the production of IL-6, a cytokine which initiates osteoclastogenesis. These in vitro findings suggest that 22K- and 20K-hGH may equipotently stimulate bone remodeling and elicit anabolic effects on skeletal tissue when administered in vivo.