For many years now, the combination trimethoprim plus sulfamethoxazole (active ingredients of Bactrim) has proved to be an effective chemotherapeutic agent with a broad spectrum of antibacterial activity against both gram-positive and gram-negative organisms, including beta-hemolytic streptococci, staphylococci, pneumococci, Haemophilus influenza, Escherichia coli, Proteus mirabilis, Proteus vulgaris, Klebsiella, Enterobacter and Citrobacter. In this comparative study, the antibacterial activity of the combination against 2,229 gram-negative and 1,338 gram-positive strains encountered in domiciliary practice was tested and compared with that of other commonly used antimicrobials. Minimum inhibitory concentrations (MICs) were determined in microtitre plates using serial dilutions. With regard to the gram-negative strains, trimethoprim + sulfamethoxazole, with an MIC90 of less than 2 mg/l for most isolates, was the most active substance apart from norfloxacin. The combination also had a powerful inhibitory effect on gram-positive strains, the MIC90 being between 2 and 4 mg/l for all strains except Staphylococcus epidermidis.