Spirosomes, very find spiral particles, were isolated from a protoplastlysate of Lactobacillus brevis ATCC 8287 by differential centrifugation and purified further by potassium tartrate density gradient centrifugation. The purified spirosome preparation showed a maximum peak around 275 nm on the ultraviolet absorption spectrum and it consisted of about 94.5% protein. The buoyant density in CsCl of the spirosomes was 1.320 g/cm3. The spirosomes were composed mainly of a single protein (spirosin with an apparent molecular weight of about 95,000 as determined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The protein of the spirosomes was found to be composed predominantly of neutral amino acids accompanied by approximately equal amounts of acidic and basic amino acids. The spirosomes showed one antigenic determinant in the immunodiffusion test. The spirosomes were readily degraded by the action or proteolytic enzymes and lost their antigenicity, but they were not affected by treatment with either deoxyribonuclease or ribonuclease. The spiral structure of the spirosome was also found to be disintegrated by treatment with 1 M guanidine hydrochloride, 4 M urea or 0.1% SDS, but not by the action of deoxycholate, nonionic detergents or mercaptoethanol, as observed in the electron microscope.