Bacterial growth kinetic experiments have been performed to study the potentiating effect of trimethoprim (TMP) and TMP derivatives on the antibacterial action of sulfonamides (SA). The TMP derivatives showed different inhibitory effects when used alone. Derivatives with decreased inhibitory activity excerted ‘bactericidal’ effects only at high concentrations. A rather fast development of bacteria with lower sensitivity against these derivatives has also been observed. In the combined action with SA, however, the same maximal ‘kill’ rate was observed for TMP derivatives as tetroxoprim or derivative IV (table I) at low concentrations of these inhibitors. Under the experimental conditions there is no significant difference between the studied TMP derivatives and various SA in the combined