The present study was performed to investigate whether the introduction of a wild-type p53 gene into human osteosarcoma cells could alter the growth rate and enhance the cytocidal effect of cisplatin (CDDP) and the synergistic antitumor effect of caffeine. The lipofection method was used to transfect a wild-type p53 expression plasmid into the human osteosarcoma cell line, Saos2, which has both p53 alleles deleted. The transfected cells, Saos2/p53, had a reduced growth rate compared with the parental cell line. The colorimetric WST-1 assay demonstrated that Saos2/p53 cells were twice as sensitive to CDDP alone at a 50% inhibition concentration than the parental Saos2 cells. Caffeine significantly potentiated the cytocidal effect of CDDP in the Saos2/p53 cells. Furthermore, the TUNEL assay revealed that following treatment both with CDDP alone and with CDDP combined with caffeine, a higher percentage of the Saos2/p53 cells underwent apoptosis than did the parental Saos2 cells. Therefore the cytocidal effect of CDDP and the synergistic antitumor effect of caffeine are enhanced by the introduction of a wild-type p53 gene into a human osteosarcoma cell line null for p53. This raises the possibility that gene therapy using the p53 gene may prove efficatious for human osteosarcomas lacking p53 and which are resistant to standard chemotherapy.