The relation between the concentration of Terramycin and the lag preceding growth has been determined for the unadapted strain and for strains of Bact. lactis aerogenes (Aerobacter aerogenes) fully adapted to a range of concentrations of drug. The family of curves so obtained, whose horizontal spacing can be predicted by a simple mathematical expression, shows that the resistance is continuously graded to the 'training' concentration. After one subculture in a low concentration of drug the resistance declines gradually, on growth in drug-free medium, to a state intermediate between full resistance and sensitivity and remains in this state for at least 500 generations in the absence of drug. Longer 'training' in which secondary effects of the drug occur results in a more stable resistance. Time-number relations for colony formation on solid media containing Terramycin show that no fully resistant forms exist in the original population. These responses are interpreted in terms of a competition between lethal and adaptive processes, and the changes in the macromolecular composition of the cells during the first subculture in liquid medium containing Terramycin are in accord with this.