We followed the evolution of DNA polymerase alpha, beta and terminal deoxynucleotidyl transferase (TdT) in the thymus, spleen and bone marrow of tumor bearing hamsters. Tumors were induced by spontaneously transformed fibroblasts (EHB), by SV40 (ZD) or by 20-methylcholanthrene (MCH2) transformed fibroblasts. We have shown that, in the thymus, the TdT activity of the various tumor bearing hamsters is generally inferior to the TdT activity of the control. In the spleen of animals bearing viral or spontaneously induced (ZD or EHB) tumors, the TdT activities were higher than in the controls. Moreover, DNA polymerase alpha and DNA polymerase beta activities in the spleen of the EHB tumor bearing hamsters were higher than in the controls. This fact, however, was not observed in the two other kinds of tumor, possibly because EHB tumors were growing much faster and led to earlier changes in DNA polymerases activities, as well as in spleen size and cellular populations. Finally, in the bone marrow, TdT, polymerase alpha and beta of ZD and EHB tumor bearers reached much higher activities than in the controls; for the MCH2 tumor bearing hamsters, no difference with the controls could be observed.