Eleven strains of Mycobacterium tuberculosis and 13 other species of mycobacteria were examined for susceptibility to 26 strains of viridans streptococci. Susceptibility was detected by the simultaneous antagonism technique, but not by tests of deferred antagonism. It is concluded that in-vitro inhibition of mycobacterial growth by actively growing streptococci is due to a peroxide-mediated antagonism which is too variable to be of value for identification. However, a standard bactericidal test with hydrogen peroxide is worth investigation because this might provide a simple inexpensive aid for the identification of M. avium-intracellulare serotypes.