The hypothesis that defective cell wall synthesis seems to represent a final common pathway of drug-induced injury either within the bacterial cell or on its surface was supported by two different findings: (1) sulfamethoxazole and trimethoprim, either alone or in combination, induced morphological findings in various Escherichia coli and Proteus mirabilis strains identical to those found after incubation with so-called cell-wall-active antibiotics (e.g., penicillin) and (2) cell-wall-defective bacteria (L forms, spheroblasts) were resistant to sulfonamides and/or trimethoprim as compared to their normal bacterial cells.