Salmonella typhi O and H antibody titres were determined by the Standard Agglutination Test (SAT) in 85 patients with bacteriologically proven typhoid, 102 patients with non-typhoidal febrile illnesses (control group 1), and 170 healthy subjects (control group 2). An O antibody titre of 1/120 occurred in 93% of patients with typhoid fever (7% false negatives), and in 17% and 12% control groups 1 and 2 respectively. An H antibody titre of 1/120 was found only in 64% of patients with typhoid (false negatives in 36%). At titres other than 1/120, the proportion of false negatives or false positives increased ie. the sensitivity and specificity decreased. Although the SAT continues to be used in Sri Lanka as the main diagnostic test in typhoid, the sensitivity and specificity of this test fall below the requirements of a useful diagnostic test.