The effect of agitation of BACTEC 13A bottles (Becton Dickinson) on the recovery of Mycobacterium avium complex (MAC) from blood was compared with that of static incubation. A total of 265 blood specimens was inoculated in duplicate into BACTEC 13A bottles. One specimen was statically incubated at 35°C, and the other was incubated with agitation on a Gyrotory shaker at 35°C for the first 2 weeks and thereafter without shaking for up to 12 weeks. Of the 265 specimens, 77 (29.1%) were positive in

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... either one or both of the paired bottles. The average detection times for the shaken and nonshaken bottles were 12.7 and 15.9 days, respectively. A total of 10.4% of the specimens in the shaken bottles became positive 1 week before those in the nonshaken bottles, and 16.9% of the shaken cultures were positive more than 2 weeks before their counterparts. A further 46.8% of the agitated specimens became positive while the corresponding nonagitated cultures remained negative. When both specimens became positive at the same time, 88% of the shaken cultures had higher growth indices than their nonshaken counterparts. A further 11 paired blood cultures were taken from patients known to be infected with MAC to assess the effect of agitation of bottles on the utility of making twice-weekly readings during the first 2 weeks of incubation. Ten of the 11 sets of specimens in the shaken bottles were positive 1 or more weeks before those in the corresponding nonshaken bottles. In the remaining set, both specimens became positive on the same day; however, the growth index of the agitated culture was higher. The results indicate that agitation of BACTEC 13A blood cultures enhances the growth of MAC in blood cultures and may lead to shorter detection times.