A Reverse Passive Haemagglutination Test (RPHA) was designed for the detection of Salmonella typhi antigen and rapid diagnosis of typhoid fever. Two per cent fresh sheep RBC's were coated with 32 micrograms/ml of immunoglobulin. The minimal detectable level of the antigen was 1250 micrograms/ml. Cross reactions were observed with the samples of patients suffering from Salmonella paratyphi A and pseudomonas infections. The RPHA established was used for the detection of S. typhi antigen in culture broths from 100 patients with clinically suspected typhoid fever with culture and/or widal positive, 50 patients with septicemia caused by bacteria other than S. typhi and 50 normal, afebrile healthy controls. It was found that the sensitivity and specificity of this assay was 70% and 92% respectively.