Following intravenous administration of 2 g ceftriaxone, concentrations of the drug were assayed in serum, in thoracic duct lymph from dogs, and in mesenteric lymph nodes in patients. Antibacterial activity of lymph against S. typhi was also studied. Results show that ceftriaxone concentrations in serum and lymph are comparable; with a satisfactory antibacterial activity of both fluids against S. typhi. In mesenteric lymph nodes, mean ceftriaxone concentration was approximately 1000 times the MIC for S. typhi. Our data contribute to explain the successful clinical results achieved with ceftriaxone in patients with typhoid fever.