A set of 12 acidic, 5 imidic, and 5 amidic sulfanilamides (SA) were tested for their inhibitory activity on dihydropteroate synthase of Escherichia coli. The enzyme inhibition indexes (EII50) were compared with the growth inhibition indexes (GII50), and electronic structures of SA and cell permeability effects were discussed as possible determinant factors of the observed variation of the activity in the SA set. The results strongly support the following conclusion: (a) permeability factors are highly effective in depressing the activity of SA in growth inhibition with respect to enzyme inhibition, but they do not appear to contribute significantly to the activity variation; (b) the activities of the different SA, both in growth and enzyme inhibition experiments, are well accounted for by the electronic features of these compounds.