A new blood culture medium (16B) containing adsorbent and cationic exchange resins has become available for use with the BACTEC instrument (Johnston Laboratories, Towson, MD). Its purpose is to enhance the detection of bacteremia through binding of antimicrobials. The performance of the BACTEC 16B resin medium was compared with the routine BACTEC 6B medium in patients with suspected sepsis receiving antibiotics. A total of 1,227 blood specimens were inoculated in 6B and 16B media and yielded 93 positive cultures from 43 clinically septic patients. Of 103 bacterial isolates recovered, 63 (61.2%) were recovered in both media, 14 (13.6%) in the routine 6B medium only, and 26 (25.2%) in the resin medium only (P greater than 0.05). Staphylococci, both coagulase positive and negative, were recovered much more frequently in resin medium (P less than 0.01). When the results of all the blood culture sets collected for each patient on any given day were considered, the routine 6B medium was the only source of isolation for seven bacterial species in six patients, and the resin medium was the only source of isolation for nine species in nine patients. However, of the nine organisms whose sole isolation source was the resin medium, eight were recovered early in the course of antibiotic therapy (6 within 24 to 36 hours and 2 within 36 to 48 hours of the first antibiotic dose) and had been isolated previously in routine 6B medium. In no instance was the antibiotic regimen changed as a result of the persistence of the organism in resin medium in the early phases of treatment. The use of resin medium did not improve overall detection time for 63 isolates recovered in both media. In conclusion, although the 16B resin medium did recover a greater number of bacterial isolates, it contributed very little information that might be of use in modifying and improving the treatment of septic patients receiving antimicrobials.