In a retrospective study of 1,143 blood culture sets, BACTEC aerobic (6B) and osmotically stabilized (8B) media were compared individually with resin-containing (16B) medium for the isolation of bacteria from the blood of patients receiving antimicrobial therapy. The 16B medium was found to detect significantly more positive cultures than either 6B (P less than 0.01) or 8B (P less than 0.001). For 22 of 25 isolates of Staphylococcus aureus from 18 patients, 16B medium provided the only means of recovery. All but one of these patients were receiving appropriate antimicrobial therapy, and 16 of 18 had a previous blood culture set positive for S. aureus that did not include a 16B bottle. There was no evidence of a change in antimicrobial therapy in response to a 16B positive culture in these patients. No significant increase in the recovery of other gram-positive or gram-negative bacteria or decrease in the time to radiometric detection of positive cultures as a result of using 16B medium was noted.