This component was partially characterized, and its major antigenic determinants were associated with anionic mucopolysaccharides. Electrophoretically at pH 8.3 it migrated anodally to albumin, its molecular weight was between 9,000 and 12,000, and it was soluble in 50% saturated ammonium sulfate. Binding to antibody was destroyed after treatment with Pronase, but not after DNase or RNase.

Inhibition of the reaction, as measured by the ammonium sulfate test, between the ¹³¹I-labeled component and antisera from rabbits that had been immunized with sonicated 5159 organisms, was studied. These experiments demonstrated a capacity to define subtle similarities and differences among different mycobacteria and mycobacterial components. Some microorganisms not clearly related to mycobacteria also partially inhibited this reaction, suggesting that they shared antigenic groups with the component derived from 5159 organisms. The studies described suggested the advisability of using direct primary tests and purified components of mycobacteria to differentiate further the antigenic groups between individual pathogenic mycobacteria and between pathogenic and nonpathogenic organisms.