We performed DNA microarray analysis on two BALB/3T3 transformants (A5 and A6) induced by polyurethane (PU) film, two (L11 and L21) induced by biodegradable poly(L-lactic acid) (PLLA) film, and the parental cells. The transforming ability of the cells was in the order A5 < A6 < L21 < L11. In all, 1176 cancer-related genes were up- or down-regulated in at least one transformant. Those that were markedly up-regulated were c-fos protooncogene, FBJ osteosarcoma oncogene B, and Jun oncogene; those markedly down-regulated were pleiotrophin, histidine triad nucleotide-binding protein, protein kinase C iota, and large multifunctional protease 7. A common function of proteins encoded by genes that underwent marked expression changes was bone formation. The genes were c-fos, FBJ osteosarcoma, Jun, pleiotrophin, a disintegrin-like and metalloprotease with TS-1 motif protein 1. This finding was consistent with the tumor formation in the 2-year PLLA or PU subcutaneous implantation into rats. The number of genes that underwent marked expression change in each transformant was consistent with its malignancy. PLLA induced more malignant transformants than PU, especially in relation to osteosarcoma-like gene expression. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res 68A: 376382, 2004 [ABSTRACT FROM AUTHOR]