1. Sonicated glycophorin-containing vesicles of dioleoyl phosphatidylcholine have been made. The outside-inside distributions of the lipid molecules in these vesicles was measured with NMR and was found to be comparable with that of protein-free vesicles. 2. The transbilayer distribution of palmitoyl lysophosphatidylcholine in these vesicles is such that they have a significantly higher content of the lyso-compound in the inner monolayer when compared with vesicles without glycophorin. 3. Lysophosphatidylcholine, added to pre-existing glycophorin-containing vesicles, is incorporated in the outer monolayer of these vesicles. Subsequently it is able to move to the inner monolayer with an estimated half time of about 1.5 h at 4 degrees C. This was measured with 13C-NMR using [N-13CH3]lysophosphatidylcholine. 4. Treatment of co-sonicated vesicles of phosphatidylcholine and lysophosphatidylcholine containing glycophorin with the enzyme lysophospholipase results in a complete degradation of the lyso-compound. A half time of transbilayer movement of lysophosphatidylcholine during this experiment was estimated to be about 1 h at 37 degrees C.