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Methods for purifying the not cell-bound complement fixing antigen (NCBA) of M. pneumoniae, present in cellfree filtrates from broth cultures of the organism, have been examined. Precipitations with ammonium sulphate (AS) and isopycnic CsCl gradient centrifugation have both proved effective methods by which to separate NCBA from broth medium proteins. Using 31 per cent AS, ab. 40 per cent of the NCBA measured in native filtrates was regained in the precipitates, the protein content per CF-unit of which was ab. 5 per cent of that in the native filtrates. With increasing salt concentrations, the protein figure of the precipitates rose far more steeply than the NCBA yield, resulting in an inverse ratio between the yield and the degree of purification. The AS precipitation method also facilitated a substantial concentration of NCBA. In the CsCl gradient centrifugation, NCBA was located in the buoyant density range 1.084–1.105 g/ml, indicating a lipid nature of NCBA. By a combined application of 31 per cent AS precipitation and CsCl gradient centrifugation, a concentrated preparation of NCBA was produced with a yield on the 30 per cent level and a protein figure below 1 per cent of that in native filtrates.