To compare the performance of the BACTEC 13A (Becton Dickinson, Sparks, Md.), BACTEC MYCO/F LYTIC (Becton Dickinson), BacT/ALERT MB (bioMérieux, Durham, N.C.), and ISOLATOR 10 lysis-centrifugation (Wampole Laboratories, Cranbury, N.J.) systems for detection of mycobacteremia in adults, we inoculated 5-ml aliquots of blood from patients with suspected mycobacteremia into the bottle or tube required for each system. Of 600 sets tested, 85 (14%) yielded Mycobacterium avium complex (MAC) and 9 (2%) yielded other species of mycobacteria. Of 26 complete (three bottles and one tube) adequately filled (5 +/- 1 ml) sets from which MAC was recovered, BACTEC 13A was positive for 19 (73%), BACTEC MYCO/F LYTIC was positive for 21 (81%), BacT/ALERT MB was positive for 22 (85%), and ISOLATOR 10 was positive for 21 (81%). Of the six possible two-way comparisons, the mean times to detection for the recovery of MAC from each bottle in positive adequately paired sets were 15.3 days for BACTEC 13A versus 12.8 days for MYCO/F LYTIC for 33 of 340 pairs, 14.1 days for BACTEC 13A versus 11.6 days for BacT/ALERT MB for 38 of 380 pairs, 12.6 days for BACTEC 13A versus 20.0 days for ISOLATOR 10 for 26 of 261 pairs, 12.8 days for BACTEC MYCO/F LYTIC versus 11.0 days for BacT/ALERT MB for 33 of 340 pairs, 13.2 days for BACTEC MYCO/F LYTIC versus 20.4 days for ISOLATOR 10 for 24 of 230 pairs, and 9.9 days for BacT/ALERT MB versus 19.0 days for ISOLATOR 10 for 24 of 257 pairs. There were no significant differences in yields between the systems. However, the mean time to detection differed significantly among the systems. The time to detection was shortest for BacT/ALERT MB, followed by BACTEC MYCO/F LYTIC and BACTEC 13A and then ISOLATOR 10. Although the numbers were too small for statistical comparison, the time to detection was substantially shorter for MAC than for Mycobacterium tuberculosis complex in the liquid systems. The continuously monitored systems (BACTEC MYCO/F LYTIC and BacT/ALERT MB) were as sensitive and, on balance, faster for the detection of MAC bacteremia than were the heretofore standard manual ISOLATOR 10 and radiometric BACTEC 13A systems.