The new resin-containing BACTEC 26 Plus blood culturing system (Becton Dickinson Diagnostic Instrument Systems, Towson, Md.) was compared with the Isolator 10 system (Wampole Laboratories, Cranbury, N.J.). Blood samples were drawn by syringe, and equal 10-ml volumes were evaluated in each blood culture system by the recommended methods. Both systems were incubated aerobically with 5% CO2. Of 11,506 acceptable study specimens, 1,788 aerobic isolates were recovered. Overall, recoveries was similar for the two systems, with 626 bacteria or fungi recovered in the BACTEC 26 Plus system only, 499 recovered in the Isolator system only, and 663 recovered in both systems. Of 345 gram-negative rods, 62 grew in the BACTEC system only and 109 grew in the Isolator system only (P less than 0.001). Thirty-three of these Isolator-only gram-negative organisms were Acinetobacter spp. Of 209 yeasts, 38 grew in BACTEC only and 81 grew in Isolator only (P less than 0.001). Of 200 streptococci and enterococci, 98 were recovered in BACTEC only and 26 grew in Isolator only (P less than 0.001). Two hundred twenty-eight independent episodes of gram-negative rod bacteremia occurred. Isolator was the first system positive in 59 of 197 episodes, compared with 45 of 197 for BACTEC when Acinetobacter episodes were excluded. Times to detection were similar for the two systems. High colony counts correlated with repeat positive blood cultures. Isolator and BACTEC had similar overall recoveries, with individual merits and deficiencies for both systems. The additional quantitative information derived from Isolator had utility in our institution.