Colicin A belongs to the group of colicins which form voltage dependent ionic channels in planar lipid bilayers. (Schein et al., 1978; Pattus et al., 1983). In vivo, the primary effects of these colicins are a leakage of cytoplasmic K+ (Wendt, 1970) and small ions (Lusk and Nelson, 1972), a decrease of internal ATP, a collapse of the electrochemical gradient of protons (Δ μH +), and consequently an inhibition of the ΔμH+-driven active transport systems (for reviews see Luria, 1975; Konisky, 1978, 1982). On the basis of these in vitro and in vivo properties, it is generally admitted that the killing activity of these colicins results from the formation of channels in the cytoplasmic membrane.