Interleukin 1(IL-1), a 17.5 KD glycoprotein, is known to be associated with local bone resorption. In the present study, we examined the effects of IL-1, compared with insulin and parathyroid hormone (PTH), on DNA, protein and collagen synthesis in UMR-106-01 rat osteoblastic osteosarcoma cells. When 200 units/mL IL-1 was administered to UMR-106-01 cells, [3H]-thymidine uptake increased to 119% of the untreated control. But when 10 nM insulin was added to the cells, [3H]- thymidine uptake increased to 130% and when 1 nM PTH was added, the uptake decreased to 89% of the control. On the other hand, protein and collagen synthesis, measured by [3H]-leucine and [3H]-proline incorporation respectively, were not affected by IL-1 administration compared to the other hormones. These results indicate that IL-1 effects osteoblast-like cells, stimulating DNA synthesis via a different mechanism to the well-known cell growth factor, insulin.