Counterimmunoelectrophoresis (CIE), Gram staining, and quantitative measurements were performed on simulated blood cultures at hourly intervals after inoculation with Streptococcus pneumoniae, Klebsiella pneumoniae, or Haemophilus influenzae. The CIE became positive either at the same time as the Gram stain or within the ensuing five hours. In no case was CIE positive when the Gram stain was negative. The numbers of colony-forming units milliliter necessary for a positive CIE result varied with the types of microorganisms. The procedure of CIE appears useful as a rapid screening technic for identification of bacteria in blood cultures when microorganisms are seen in Gram-stained smears.