The effects of Corynebacterium parvum on the lymphohemopoietic tissues of mice was investigated by H3-TdR metabolism, organ weights, DNA-polymerase-alpha activity and terminal deoxynucleotidyl transferase (TdT) activity. Marked increase in spleen size occurred. The increase in size was accompanied by increases in DNA-P-alpha activity and H3TdR uptake. This indicated that the spleen was a major site of cell proliferation and of increase in population size after C. parvum stimulation. Thymus and bone marrow changes are also described. The thymus showed a marked decrease in size which was maximal in 10 days and showed recovery thereafter. Thymus TdT activity fell immediately and may reflect either a release or lysis of thymocytes which contain TdT activity. However, no change in TdT activity was measurable in the bone marrow or spleen. The principal cell population increase had phagocytic activity and was presumably monocytes-macrophages, and an increase in such cells was found in the spleen.