Growth in the presence of diphenylamine caused complement resistant strains of Escherichia coli to become complement sensitive. Associated with this increased sensitivity was a fall in the amounts of specific O and K antigens extracted from the bacteria. Synthesis of the basal core of the O antigens was also affected. Nevertheless, there was an increased uptake of O antibody. Possible mechanisms by which complement sensitivity could be induced by diphenylamine are removal of the inhibitory effects of K antigen, and exposure of hidden antigenic determinants in the O antigen.