In 21 persons with previous typhoid fever and in 15 controls, thymidine incorporation of blood lymphocytes stimulated in vitro by killed S. typhi was studied. The optimal culture conditions were established to be a cell density of 105 cells per vial, stimulated with 108 S. typhi and incubated for 5 days. The lymphocyte response to S. typhi was significantly higher in the typhoid group than in the controls. The lymphocyte responsiveness was not correlated to the time elapsed since the attack, and was not found different in patients who had typhoid relapse. No difference was found in lymphocyte response to the mitogens PHA [phytohemagglutinin], PWM [pokeweed mitogen] and Con[concanavalin]A between the typhoid group and the controls.