The aggregation form of mixtures of phosphatidylcholine and MDP derivatives was studied. The mixed vesicles containing less than 40% of B30-MDP were unilamellar, and over 40% B30-MDP aggregates of mixed vesicles or oligolammellar vesicles appeared. The mixed vesicles containing less than 20% of DV-7401 were unilamellar, and over 20% of it fibriform structures appeared.