The effects of the novel tetramethylpiperidyl (TMP)-substituted phenazine, B4128 (0.6-2.5 mg/L), on growth, phospholipase A2(PLA2) activity, cation (K+, Ca2+) fluxes and energy metabolism (ATP) of Mycobacterium aurum A(+) and Mycobacterium tuberculosis (H37Ra) have been investigated in vitro. Growth, PLA2 and cation fluxes were measured radiometrically, while microbial ATP was assayed by means of a luciferin/luciferase chemiluminescence method. Exposure of the mycobacteria to B4128 resulted in immediate, dose-related enhancement of microbial PLA2 activity and inhibition of K+ influx, which preceded effects on microbial ATP, influx of Ca2+ and growth. These results demonstrate that B4128 possesses membrane-directed antimycobacterial properties that are similar to those of clofazimine.