Three different types of response to a bacterial growth inhibitor can be distinguished.In type 1 as exemplified by 5-bromouracil, the plot of log density against time is linear in agreement with the exponential growth law.The slopes of the curves, which are proportional to the rates of division, are altered by the inhibitor in the manner expected for inhibition of a reaction sequence in the steady state.The lack of delay in attaining the final growth rate suggests that the rate of penetration of such compounds into the cell is not a limiting factor in the degree of inhibition.Type 2 response, as represented by sulfanilamide, is more complex.The final growth rate is not reached until the cells have undergone 2 or 3 divisions.Perhaps the reason for the delay is that even if the synthesis of some enzyme is stopped, the existing mols. continue to function until destroyed by catabolic processes, or are diluted below an effective concentrationOr possibly very slow diffusion into the cell may be the explanation.The type 3 reaction, represented by pyridine-3-sulfonic acid, is still more complex.Results here depend markedly upon size of inoculum.No satisfactory explanation of this type of inhibition is known.