Baugh, C. L. (Fort Detrick, Frederick, Md.), A. W. Andrews, and M. J. Surgalla. Effects of bicarbonate on growth of Pasteurella pestis. III. Replacement of bicarbonate by pyrimidines. J. Bacteriol. 88:1394–1398. 1964.—The effect of carbon dioxide on the growth of virulent Pasteurella pestis cultures at 37 C with aeration was studied by substituting known products of carbon dioxide fixation for bicarbonate in the test system. The growth of the virulent cells in the inoculum is stimulated and the culture remains virulent, if bicarbonate is replaced by orotic acid. The addition of cytosine, uracil, or citrulline also results in the retention of virulence, but the effect on the growth of the virulent cells is not as pronounced as with bicarbonate or orotic acid. It is proposed that an impaired pyrimidine synthesis due to a deficiency in carbomyl phosphate is responsible for the loss of virulence by P. pestis in aerated broth cultures at 37 C. The carbamyl phosphate deficiency may be enhanced by the loss of metabolically produced carbon dioxide at 37 C.