Thirty-four primary lung cancers were analyzed for abnormalities in the myc family geners (c-myc, N-myc, L-myc), using the Southern blot hybridization method. They were subcutaneously transplanted into nude mice. The Southern blot analysis showed that c-myc and L-myc genes were amplified in 4 non-small cell carcinomas and 3 small cell carcinomas respectively. Allelic deletion of the L-myc gene was observed in 7 cancers, including 2 carcinomas which also had an additional band of the c-myc gene or amplification of the L-myc gene. No abnormalities in the N-myc gene were observed in this study. Of 13 cancers with abnormalities in the myc family genes, 11 including all tumors with myc gene amplification were transplantable to nude mice. Of 21 tumors without any abnormalities in the myc family genes, however, only 6 were transplantable to nude mice (p<0.005). These results indicate that abnormalities in the myc family genes, especially gene amplification might promote tumor-forming capacity in xenotransplantation of lung cancers and this phenomenon might be closely related to the function of the myc gene.