The contribution of serum adenosine deaminase (ADA) activity to the diagnosis of typhoid fever was assessed in 246 children and in 46 adults, by Giusti's original technique. Children included otherwise healthy patients admitted for elective surgical conditions or under follow up for epilepsy which were considered to be a control group (n: 81), presumptive viral diseases (n: 31), miscellaneous febrile diseases except for typhoid fever (n: 41), different kinds of bacteremia (n: 6), diarrhea due to Salmonella typhimurium (n: 14), viral hepatitis (n: 24), and culture proven typhoid fever (n: 49). Adult's group included 39 healthy controls and 7 patients with culture proven typhoid fever. Among children mean ADA activity was as follows: control group 28 +/- 7.8, viral disease 35.3 +/- 13.1, miscellaneous febrile disease 36.1 +/- 15.6, bacteremia group: 30.3 +/- 10.3, salmonellosis group 51.6 +/- 9, hepatitis group 68.3 +/- 34.5, typhoid fever group 124.4 +/- 40.8 U/I 37 degrees C. Among adults, values were 18.4 +/- 7.5 for controls and 112.8 +/- 19.2 U/I 37 degrees C in typhoid fever patients. In both adults and children ADA activity was significantly higher in the typhoid fever group (p < 0.0001). Untreated typhoid fever patients had their higher ADA activity between 10th and 15th day of illness. When ADA cut point was set at 80 U/I, sensitivity of the test was 91.8% and specificity was 91.4% as a preliminary clue to the recognition of typhoid fever.