The zeta potential of cells of an attenuated vaccine Mycobacterium bovis, BCG, Tice substrain, was measured over a pH range of 1.5-11.0 at low electrolyte concentration. There was a marked electro-positive charge at low pH, the zero charge point, pH 4.4, being similar for viable and heat killed vaccine. At pH 6.5 the addition of sodium dodecyl sulphate or sodium tauroglycolate made little difference to the zeta potential. However, progressive addition of cetylpyridinium chloride reduced and finally reversed the charge to a maximum of +60 mV at a concentration of 1.5 x 10(-2) mol dm-3 surfactant. Higher concentrations reduced the charge although it remained positive. The nature of the adsorbing species on the cell surface is discussed since the Tice substrain has both cationic and anionic surface charges whereas the Glaxo strain is reported to only possess electro-negative phosphate surface groups. Some hydrophobic interaction involving lipid within the surface may also be involved.