The antimicrobial activities of sulfamethoxazole (SMX) and trimethoprim (TMP), alone and in combination, were tested in 449 strains of micro-organisms freshly isolated from clinical materials. The following results were obtained :
1. Of the 449 strains investigated, approximately 60 % of the organisms were found to have MIC of SMX greater than 100 mcg/ml, whereas MIC of TMP was found to be between 0.2 and 0.78 mcg/ml in 71% of the strains.
2. Antimicrobial activities by the combination of SMX and TMP as assessed by checker board titration method were demonstrated potentiation in 66.6 % of the materials. Of particular interest was the evidence of potentiation in 44.2 % of those microorganisms which were found to have MIC greater than 100 mcg/ml of SMX alone.
