Lymphoid cells were isolated from mouse mammary tumors by isokinetic gradient centrifugation. ϑ-bearing and ALS-sensitive cells were always the largest subpopulations found in these tumors. In fractions 4 to 6 from the isokinetic gradient, ϑ-positive cells ranged from 8 to 46% in autochthonous tumors and 14 to 47% in passaged tumors, and ALS-positive cells ranged from 34 to 46% in autochthonous tumors and 14 to 100% in passaged tumors. SIg-positive cells were only found in occasional tumors and, where present, averaged 2% of the separated population. Fc receptor-bearing cells averaged 15% and phagocytic cells averaged 8% of the cells found in fractions 4 to 6 of the gradient. Occasional ALS-positive and one-third of the Fc receptor-bearing and phagocytic cells also separated in fractions 7 to 10 of the isokinetic gradient. Although the individual tumors were very variable in the proportions of the various lymphoid subpopulations that they contained, this variability was a stable characteristic for at least one passage generation.