Young strain A litter-mate mice were divided into 4 groups and fed Purina dog chow, high cystine-low protein diet ad lib., high cystine-low protein diet restricted in amt., and a low cystine-low protein diet ad lib., respectively. Differences were noted in incidence of pulmonary tumors from group to group. [female] mice on each of the 3 exptl. dietary regimens exhibited a singularly constant incidence of pulmonary tumors from group to group, each differing from that in the chow-fed controls to the same degree. The difference between the pulmonary tumor incidence in [male][male] fed the high cystine-low protein diet ad lib. and that of [male][male] fed the same diet restricted in amt. may be attributed to a difference in caloric or protein intake, or possibly to some other factor. Cystine per se within the limits employed in these diets did not affect the genesis of pulmonary tumors in strain A mice at an avg. age of 15 mos.