During the period of 18 days after transplantation the uptake of 3H-thymidine into DNA was significantly increased in the spleen of Ehrlich ascites tumor-bearing Swiss and NMRI mice. Adenosine deaminase activity was within the control limits in tumor-bearing Swiss mice and significantly increased for the tumor-bearers of the NMRI strain. However, the increase in the DNA content, which is associated to splenic hyperplasia, was evident only for the Swiss strain. The time course of the retention of 3H-DNA revealed that the half life of spleen cells ranged from 41 to 46 h in normal and tumor-bearing Swiss mice and normal NMRI mice. For tumor-bearing NMRI mice the rate of cell depletion was significantly increased (t1/2 = 32h) compared with that found in normal NMRI mice (t1/2 =44 h). The mean survival time was 23.0 days for the tumor-bearing Swiss mice and 24.6 days for the NMRI mice. These data suggest that the immune responses were similar in both strains of mice, although cell kinetics was different in the spleen of tumor-bearing Swiss and NMRI mice.