The authors investigated the antituberculosis and antitumour immunogenicity as well as tuberculin allergenicity of the lipid fractions from Mycobacterium bovis BCG strains of Danish, French, Japanese origin and of Czechoslovak 725. The fractions explored included phospholipids, Cord factor, ethanol-extractable lipids, waxes A, B, C + D and fats. The fractions were divided into three groups according to their effectiveness. 1. The Cord factor and phospholipids from all the studied strains were effective in the antituberculosis and antitumour models with the only exception of strain 725 phospholipids. Phospholipids from all strains were capable of inducing tuberculin allergy. 2. In the second group (waxes A, C + D and lipids extractable by ethanol) a variance was observed in the antigenic properties of identical fractions from different strains suggesting differing metabolism in the strains producing these fractions. A mixture of waxes C + D from the French and Danish strains showed a degree of suppression in its antituberculosis effectiveness. 3. Waxes B and fats were entirely ineffective in the antitumour model and, with the exception of waxes B from strain 725 and fats from the Japanese strain, in the antituberculosis model. The antituberculosis and antitumour effectiveness directly depended on the content of a mycolic acid complex in fractions. Tuberculin allergenicity was associated with the intensity of phospholipid production by mycobacteria.