To elucidate the effects of interleukin-1β (IL-1β) on osteogenic protein-1 (OP-1) gene expression in a polylayer culture of rabbit articular chondrocytes, we measured rabbit OP-1 mRNA using quantitative TaqMan reverse transcriptase-polymerase chain reaction (RT-PCR) techniques. Rabbit articular chondrocytes were isolated and cultured in minimum essential medium eagle α modification containing 10% fetal bovine serum for 7 days. IL-1β was then added and cultures were continued for 48 or 96 hours. OP-1 gene expression was detected in cell cultures both with and without addition of IL-1β. However, the level of expression was very low in the control group. OP-1 gene expression was significantly increased about 450- to 800-fold in IL-1β-treated groups (0.1, 1, and 10 ng/ml) versus the control group. Evaluation of serial changes in OP-1 expression after addition of IL-1β (10 ng/ml) revealed that OP-1 gene expression increased rapidly after addition of IL-1β, reaching a peak at 48 hours, and then decreasing. Simultaneous assay of CD44 expression demonstrated a rapid increase, similar to that of OP-1 expression, following addition of IL-1β: this was followed by a more gradual increase. Assay of hyaluronan synthase-2 (HAS-2) expression following addition of IL-1β showed an increase after OP-1 expression had already reached a peak. Our results demonstrate that OP-1 expression is induced by IL-1β and suggest that this expression, like that of HAS-2, may play a role as a protective mechanism against inflammatory cytokines.