Co-trimazine (sulphadiazine, 410 mg + trimethoprim, 90 mg) is a new drug combination developed especially for use in the treatment of urinary tract infections. In cross-over experiments in volunteers receiving daily doses of co-trimazine (2 X 500 mg and 1 X 1000 mg), co-trimoxazole (2X960 mg), or nitrofurantoin (3X50 mg), the degree of antibacterial activity of co-trimazine in the urine was at least as high as that of co-trimoxazole and much higher and more consistent than that of nitrofurantoin. In further cross-over experiments in volunteers receiving co-trimazine 2X1000 mg or co-trimoxazole 2X960 mg for four days no or only slight activity was found in the urine against a sulphonamide-resistant Group D streptococcus, but distinct synergistic activity between the components was found against four Escherichia coli strains sensitive to trimethoprim and sensitive or resistant to sulphonamides. Against three of the latter strains the degree of activity in the urine was higher after co-trimazine than after cotrimoxazole. The synergistic action of trimethoprim and sulphonamide in the urine appeared greater with the former than with the latter combination.