Sulfonamide-sensitive cells of Escherichia coli were incubated under standardized conditions with various sulfonamides and the quantity of sulfonamide uptake into the bacteria measured. The test substances reached a higher concentration inside the bacteria than in the incubation medium, the degree of accumulation varying for the individual substances despite uniformly applied incubation conditions. In all cases a pH dependency was detected which varied with the pKa values of the substances tested. On the basis of the pKa dependency of the pH effect, the sulfonamide uptake can be interpreted as being a passive diffusion process. Knowing the intracellular pH value, which was determined by the DMO method, it was then possible to calculate the theoretically expected value for the accumulation as a concentration coefficient. Since the calculated and experimentally determined values differ approximately by a factor of 2, an intracellular binding of the sulfonamides is assumed. The same results were obtained in cells of sulfonamide-resistant E. coli strains.