By use of quantitative histochemistry, effects of interleukin 1 (IL1) on the differentiation of osteoblasts derived from neonatal rabbit calvaria were studied. The results showed that IL 1 inhibits the alkaline phosphatase activity in the cultured osteoblasts. Also, both quantitative immunohistochemistry study and radioimmunoassay study showed that IL-1 has the ability to stimulate osteocalcin production and secretion in the cultured osteoblasts. Our results suggest that IL 1 plays a role in the regulation of osteoblastic differentiation, and in the end inhibits the calcification of bone matrix.