The action of retinol and carotenoids on bone cells was investigated in vitro by evaluating cell growth, alkaline phosphatase activ-ity and the mRNA expression of a differentiation marker protein of osteoblastic cells. The clonal osteogenic cell line MC3T3-El, established from newborn mouse calvaria, has a capacity of differentiation into osteoblast and mineralization in vitro. Retinol and β-carotene inhibited the proliferation of MC3T3-E1 cells as well as DNA synthesis of the cells in a dose-dependent manner. Retinol induced differentiation of the MC3T3-E1 cells, by increasing alkaline phosphatase activity dose depen-dently, in a range from 1 to 100nM. Beta-carotene increased alkaline phosphatase activity in a dose-related manner in a range from 0.1 to 5μM. Osteopontin is one of the matrix proteins which osteoblasts produce. Retinol increased the expression of osteopontin mRNA in a range from 1 to 100 nM. Beta-carotene also increased osteopontin mRNA expression, reaching a plateau at 1μM. The induction of differentiation of MC3T3-E 1 cells by β-carotene was dose-dependent but was two orders of magni-tude less active than that produced by retinoids. Within the MC3T3-El cells, part of the β-carotene was effectively converted into retinol. Alpha-carotene, canthaxanthin and lycopene also inhibited cell proliferation at μM and increased alkaline phosphatase activity and osteopontin mRNA expression, but less potently so than β-carotene. Retinol and carotenoids were concluded to have a direct stimulatory effect on the differentiation of osteoblasts at the physiological concentration.