Bone matrix gelatin (BMG) is the insoluble extract with osteoinductive capacity, obtained during the intermediate step of bone morphogenetic protein (BMP) extraction. The effect of BMG on the rate of DNA synthesis was assessed in the primary cultures of osteoblast from fetal rat calvariae by the rate of 3H-thymidine incorporation into DNA. At the concentrations of 15–60 ng/ml for 24 hours of incubation, BMG stimulated the incorporation of 3H-thymidine by 127–165% in primary osteoblast cultures. Dose-response curve was obtained with the peak response at 30 ng/ml of BMG. We also assessed the effect of BMG in the famous osteoblast cell line, that is, MC3T3-El cells, and the similar increase in 3H-thymidine uptake was obtained at the lower doses of BMG. To compare the effect of BMG on the osteoblast with that on the periosteal cells, such as fibroblast, the uptake of 3H-thymidine was measured in NRK fibroblast, the uptake of 3H-thymidine was and found that 2.5–30 ng/ml of BMG caused the increase in uptake by 160–220%. These studies indicated that BMG, which has high level of BMP activity, stimulates DNA synthesis and cell replication in the isolated osteoblasts and in MC3T3-El cells, that is periosteum-free differentiated bone cells, as well as in the osteoprogenitor periosteal cells, such as fibroblasts and undifferentiated mesenchymal cells.