The biliary excretion of tetroxoprim (TXP) was studied in 10 patients with T-tube drainage. A mean biliary peak of 32.56 +/- 7.69 micrograms/ml was observed 2-3 h after a single oral dose of 200 mg. The biliary elimination rate constant (kb) of TXP was found to be 0;056 (h-1), indicating a longer elimination from the bile than from the central blood compartment. Taking into account the TXP serum levels, following the same oral dose as reported in the literature, concentration factors of 6-8 can be calculated in the bile.