The aim of this study was to investigate whether 4-tert-octylphenol (OP) affects the differentiation of multipotent C3H10T1/2 cells, a cell line established from mouse embryonic connective tissue, into osteoblast and adipocyte lineages. Confluent C3H10T1/2 cells were incubated for 7 days with (OP-treated cultures) or without (control cultures) 15 μg/ml of OP. The 7-day treatment of confluent cells with OP decreased alkaline phosphatase activity by 81%, inhibited the expression of transforming growth factor β2, and inhibited the morphological changes in cells to an osteoblastic appearance. These results indicate that the 7-day treatment of confluent C3H10T1/2 cells with OP inhibited their differentiation into osteoblasts. Since this treatment strongly induced the expression of peroxisome proliferator–activated receptor r (PPARr) but did not stimulate triacylglycerol (TG) accumulation in cells, C3H10T1/2 cells in the control and OP-treated cultures were incubated for 2 days with a hormone mixture (insulin [INS], dexamethasone, and 1-methyl-3-isobutylxanthine) and incubated for an additional 5 days with INS alone. The TG and adiponectin contents of the OP-treated cultures were 4.2 and 4.1 times higher, respectively, than those of the control cultures. There were many more Oil Red O–staining cells in the OP-treated cultures than in the control cultures. The expression of PPARr in the OP-treated cultures was higher than that in the control cultures. These results indicate that the OP-treated cultures contained a larger number of adipocytes than the control cultures. In conclusion, treatment of C3H10T1/2 cells with OP inhibited osteoblast differentiation, causing a lineage shift toward adipocytes.