Changes in radiation sensitivity of human osteosarcoma cells after p53 introduction. Human osteosarcoma SAOS-2 cells, which have a deletion in p53 gene, were transfected with plasmid pMSVneop53 containing human p53 cDNA and neomycin-resistance gene. Three clones (SAOS-MC10, SAOS-MC11 and SAOS-MC43) among 60 clones expressed p53 mRNA. No p53 protein was observed in SAOS-MC10, while SAOS-MC11 and SAOS-MC43 produced p53 protein. The molecular weight of p53 protein in SAOS-MC43 was lower than that in SAOS-MC11, SAOS-MC11 and SAOS-MC43 were more sensitive and more resistant, respectively, to ionizing radiation than the parental SAOS-2. We suggest that exogenous p53 protein might be one of the factors determining cellular radiosensitivity.