The osteoblastic function of mouse preosteoblastic MC3T3-E1 cells, as measured by alkaline phosphatase activity and osteocalcin secretion, decreases after serial passage. To uncover genes responsible for decreased osteoblastic function in high-passage cells, we have studied passage dependent change of gene expression in MC3T3-E1 cells. Changes in the expression pattern of 2000 selected genes were examined simultaneously by comparing mRNA levels between MC3T3-E1 cells at passage 20 and passage 60 using the cDNA microarray analysis. Significant changes in the steady-state abundance of 27 mRNAs were observed in response to different passage numbers, including 17 known genes, 4 ESTs with homology to known genes, and 6 genes with no previously described function or homology. Northern blot analysis was used to verify and quantify the expression of selected genes, and revealed a significant higher level of up- and downregulation compared to microarray data. These re suits indicate the existence of a significant change in gene expression in osteoblastic cells undergoing serial passages. Such changes might be responsible for a reduction in bone regeneration in older osteoblasts. Potential roles of selected genes in bone aging are discussed.