In order to elucidate the role of lysolecithin in membranes, we have examined the effect of lysolecithin on the structure and permeability of lecithin bilayer membranes. Small L-alpha-dimyristoyllecithin (DML) vesicles with myristoyllysolecithin (MLL) incorporated as well as small L-alpha-dipalmitoyllecithin (DPL) vesicles with palmitoyllysolecithin (PLL) were studied by nuclear magnetic resonance (NMR) methods at temperatures both above and below the alpha-gel in equilibrium liquid crystalline phase transition temperature (Tc) and as a function of the concentration of the incorporated lysolecithin. Europium (III) ion was used as a probe to measure the permeability of the vesicular bilayer membrane. At temperatures below Tc, these vesicles were found to be extremely permeable to europium (III) ions. The ion translocation was found to be too fast to be measured by the NMR method under these conditions. However, above the phase transition temperature the ionic permeability decreases to a rate which could be conveniently monitored, and the permeability was shown to increase with temperature and lysolecithin concentration. Analysis of the lysolecithin concentration dependence suggests the formation of ion channels within the lipid bilayer involving four lysolecithin molecules. The data below Tc suggest a phase separation below the phase transition temperature of the host lipid, leading to the formation of patches of lysolecithin molecules within the lecithin matrix. These lysolecithin clusters are presumably long-lived under these conditions and are sufficiently structurally perturbed or disordered to serve as channels for rapid ion permeation.