A study was performed with simulated blood cultures to evaluate the production of 14CO2 as an index of bacterial growth. With a range of inoculum sizes from 4 to 4,250 colony-forming units, it was not possible to detect 14CO2 within 6 hr after inoculation in 59 separate experiments. In a limited trial with patients9 blood cultures, the radiometric method failed to provide any earlier evidence of bacteremia than did routine broth cultures.