Sulfonamide-sensitive and resistant Neisseria meningitidis of serological Groups B and C bound similar amount of 35S labeled sulfadiazine. Uptake of the radioactive sulfadiazine was dependent on sulfonamide concentration but not dependent on exposure temperature. The binding of the labeled drug appeared to be firm. Elaboration of excessive amounts of p-aminobenzoic acid by sulfon-amide-resistant neisseria does not account for sulfonamide fastness. The presence of p-aminobenzoic acid does not limit the uptake of 35S sulfadiazine by sulfon-amide-sensitive or resistant neisseria.