Syrian hamster ernbryo（SHE） cells are irradiated by a particles emitted from Plutonium-238 in vtiro.Transfection frequencies of donor DNA derived from passaged cells

including monocloned cells from 20th passage cells

after irradiation are parallel with their tumorigenicity of donor cells. After trans formation

expressions of H-ras and c-myc genes are in the state of activation

which might be resulted from occurrence of mutation in H-ras gene and hypomethylation of c-myc gene respectively

even though no alteration was observed on their restriction fragment length polymorphisms（RELP） patterns of both genes. On the other hand

expression of p53 gene is also found to be in the state of activation in transformant

but no heavy staining of p53 protein appeared in the transformant-derived tumor with p53 specific McAb HD 200. This might be result from the partlal deletion of p53 cDNA in transformant by reverse transcriptional polymerase chain reaction（RT PCR）. These results imply that activation of H-ras and c-myc genes coupled with inactivation of p53 turnor suppressor gene play important role in a particle-induced cell trans formation.