We have developed a 4-h radiometric method to predict the bacteriostatic endpoint by the tube dilution method. A mixture of [U-14C] glucose, [guanido-14C] arginine, and [U-14C] glycine was used to monitor the metabolic activity of both gram-positive and gram-negative organisms. The tube dilution method and the radiometric method were performed in parallel in 18 clinical serum samples. In the radiometric method, the samples were separated into control and test portions and serially diluted in Mueller-Hinton broth. Antibiotics in the control portion were inactivated with penicillinase or cellulose phosphate or both. The radioactive mixture and a 1-h culture of the patient's infecting organisms were added to all vials. The 14CO2 production after a 3-h incubation at 37 degrees C was measured, and the percent inhibition was determined for each vial by using the control vials as reference (no inhibition). Radiometric dose-response curves obtained for all samples showed that, by using a greater than or equal to 60% inhibition at 1:8 dilution, the radiometric method correctly predicted the outcome of the tube dilution method in 16 or 18 clinical samples.