To better understand the mode of action of monochloramine, its effect on the structural and physiological integrity of Escherichia coli B was studied. The disinfectant did not severely damage the cell envelope of the bacterium or affect nucleic acid function. Inhibition of typical protein-associated biological activities, e.g., bacterial transport, respiration, and substrate dehydrogenation, were observed at concentrations normally used for disinfection. Differences in rates of kill and inhibition of various biological activities suggest that the shoulder formation on inactivation curves was due to reactions at many sensitive sites in the bacterium, the extent of which determined whether the organism remained recoverable. Includes 47 references, table, figures.