When Bact. lactis aerogenes (Aerobacter aerogenes) was grown in a synthetic medium in the presence of proflavine, the most significant effect was to prolong the lag phase. Other effects noted were the formation of abnormally long cells unless a young inoculum had been used, and a decrease in the max. population which the medium would support. Adaptation to 43 ppm. of proflavine occurred after a single passage using a young inoculum and after 5 passages when an old inoculum had been used. Adaptation to one conc. of proflavine did not confer any degree of adaptation to a higher conc. Filtrates from old cultures cancelled the effect of proflavine on the lag phase but not on the division of cells. Similar effects were obtained with regard to adaptation of the same species of bacteria to 35 ppm. of methylene blue and to 5.7 ppm. of crystal violet, with the exception of the abnormally long cells which were not observed in these cases. Adaptation to proflavine also resulted in adaptation to methylene blue and vice versa; adaptation to crystal violet did not result in any noticeable adaptation to proflavine. Adaptation to sulfonamide did not result in any adaptation to proflavine. Two coupled enzyme systems were proposed to explain some of the salient facts of bact. adaptation. The adaptation could occur under opt. conditions during the first few cell divisions. The rate of adaptation was not retarded either by an increased acidity of the medium or by the accumulation of the lag-removing diffusible cell product. Conditions of growth which favored the formation of abnormally long cells seemed to retard or to reverse growth.