The present invention provides a lyophilized formulation comprising bacterial cell wall skeleton, an oil and a surfactant, characterized in that it comprises an antioxidant; for example, an oil-in-water emulsion formulation comprising BCG-CWS and an oil and a lyophilized formulation thereof which contains tocopherols as an antioxidant and further a buffer agent consisting of phosphate, whereby the pH of an oil-in-water emulsion obtainable by rehydrating said lyophilized formulation in water is adjusted to pH 5.5 - 8.5.