Strain differences in resistance to urethan-induced lung tumors were first noted by Cowen (J:232) who showed that the major part of the difference between the susceptible A and resistant C57BL strains was due to a single locus with susceptibility dominant. In a more extensive analysis, Bloom and Falconer (J:13102) found the same strain distribution of susceptibility and confirmed the single-locus difference between strains A and C57BL. The recessive gene for resistance in C57BL was designated ptr (J:232). The ptr gene may be identical with the resistance allele at the Pas1 locus described by Malkinson et al. (J:8075).