The effects of electroporation temperature, biochemical pretreatment of cells and stage of culture on electroporation efficiency for slow-growing mycobacteria were investigated. The efficiency of transformation into Mycobacterium tuberculosis, Mycobacterium bovis and Mycobacterium intracellulare increased markedly with temperature. In contrast, the efficiency of transformation into Mycobacterium smegmatis, a fast-growing species, was higher at 0 degree C and decreased with temperature. While stage of culture had little effect, a further increase in efficiency of 2-4-fold was obtained following glycine or ethionamide pretreatment. Electroporation at 37 degrees C has been chosen as a standard condition for slow-growing species as it usually resulted in a transformation efficiency several orders of magnitude higher than that obtained at 0 degree C.