A cosmid library,constructed from DNA of the γ-radiation-transformed REC: myc cell line, designated REC:myc:γ33.was transfected into NIH/3T3 cells,yielding foci.Another round transfection of DNA from the first round focus into fresh NIH/3T3 cells produced second round foci.An active N-ras gene which originated from rat REC: myc:γ33 cells was detected in the NIH/3T3 secondary transformants.With PCR and direct DNA sequencing techniques,rat N-ras gene was found activated in the REC:myc:γ33 cells by CAA→CGA point mutation at codon 61, but not in the REC:myc cells. Also rat N-ras gene was identified as a point mutated gene in the NIH/3T3 transformants, and the endogenous N-ras gene in the NIH/3T3 recipient cells remains normal. What was found to be more interesting is that five out of six γ-radiation transformed REC:myc cell lines bear the same point mutation (CAA→CGA) indicating association of γ-radiation-induced transformation with point mutation in the N-ras gene.