The activity of two carbapenem antibiotics, meropenem and imipenem, against Mycobacterium species was assessed by means of the Bactec 460 radiometric apparatus. This system allowed a result to be read in 4-10 days. The instability of imipenem, particularly, in the test system necessitated the development of a regimen of daily addition of antibiotic to permit a more accurate assessment of antibiotic activity to be made. When antibiotic-containing media are incubated for long periods, as is the case when determining antimycobacterial activity, the stability of test antibiotics is an important factor that must be considered.