Corynebacterium parvum induces rapid proliferation of spleen cells with concomitant rapid increases in DNA synthesis as measured by [3H]thymidine uptake and increase in DNA polymerase activity. Cell number increased exponentially over 10 days. DNA polymerase activity increased 8-fold after C. parvum stimulation. The rapidity of response indicated a population response to the stimulus, and the data are consistent with a direct stimulation of spleen cell proliferation by C. parvum.