Quantitative blood cultures were obtained from 42 patients with acute Yersinia pestis infection to determine whether the concentration of bacteria in blood influenced the clinical severity and outcome of illness. In 17 bacteremic patients, colony counts in blood cultures ranged from 102/ml died, and two patients survived episodes of hypotension. Results from plasma limulus tests were positive at the time of admission in three of 10 patients tested, and these three patients had bacteremia with colony counts of >102/ml, Meningitis developed in three patients and pneumonia in two patients; these five patients all had buboes in the axillary region. Endotoxin was detected with the limulus test in the cerebrospinal fluid in the three patients with meningitis. Ten patients randomly assigned to receive streptomycin or trimethoprim-sulfamethoxazole survived. Those treated with streptomycin had a shorter median duration of fever and a lower incidence of complications than did the patients treated with trimethoprim-sulfamethoxazole.