Concentrated mixtures of lipid vesicles and pigeon erythrocyte membrane were cosonicated in order to produce functional hybrid vesicles. From the properties of the resulting material, we conclude that hybrids were very probably formed. These properties were as follows: (i) The presence of membrane increased the sonic fragmentability of lipid vesicles. Sonic fragmentability was assessed by measuring sonication-induced release of previously trapped [14C]-choline and trapping of external [3H]-choline. (ii) Space enclosed by lipid was served by the membrane-like properties of 36Cl- permeability and ATP-dependent 45Ca++ uptake activity. (iii) 36Cl-permeability was more readily and fully induced into the more easily fragmented lipid vesicles. Further sonication caused loss of the induced 36Cl--permeability. This loss was less rapid with the less easily fragmented lipid vesicles; i.e., less easily fragmented lipids protected 36Cl--permeability better. (iv) Glycine uptake activity was partially protected from sonic damage by the presence of lipid vesicles. (v) On centrifugation in bovine serum albumin density gradients, cosonicated material showed lipid properties (enclosed choline and 32Pi space and [3H]-cholesterol) and membrane properties (36Cl--permeability and ATP-dependent 45Ca2+ uptake) coinciding at a density intermediate between those reached by separately sonicated membrane and lipid vesicles. (vi) Electron micrographs showed the disappearance of pure membrane-like structures and the appearance of large amounts of new vesicles whose appearance is consistent with a hybrid structure.