p53-independent induction of WAF1/Cip1 is correlated with osteoblastic differentiation by vitamin D3. 1Alpha,25(OH)2 vitamin D3 (1,25(OH)2D3) can induce differentiation of osteoblastic cells by arresting the cell cycle at G1. The p53-inducible gene, WAF1/Cip1, is one of the inhibitors of cyclin-dependent kinases and can inhibit the phosphorylation of retinoblastoma protein (pRB), thereby keeping pRB functionally active. Here we show that in a p53-null human osteoblastic osteosarcoma MG-63 cell line, 10 nM of 1,25(OH)2D3 completely inhibits cell growth and increases alkaline phosphatase activity, which suggests the induction of osteoblastic differentiation. We also found a p53-independent increase of WAF1/Cip1 mRNA and promoter activation by 1,25(OH)2D3. On the other hand, the expression and the promoter activity of the RB gene decreased after treatment with 1,25(OH)2D3 during the differentiation of MG-63 cells. Our results suggest that the p53-independent WAF1/Cip1 induction by 1,25(OH)2D3 is important for osteoblastic differentiation of MG-63 cells.