An enzyme-linked immunoassay was developed for diagnosing enteric fever. The test measured the inhibition of binding between a labelled, monoclonal IgM antibody and the insolubilized antigen, Salmonella typhi lipopolysaccharide (LPS). Good discrimination was seen between 32 proven typhoid cases (88.0 +/- 4.4% inhibition) and non-typhoid cases. The latter consisted of 27 febrile patients bacteriologically and serologically (Widal test) found to be negative for typhoid (26.3 +/- 10.8% inhibition), 46 patients screened for syphilis (VDRL test) but found negative (31.2 +/- 13.3% inhibition), and 27 healthy blood donors (44.6 +/- 13.9% inhibition). The test also efficiently detected all 5 known typhoid carriers (90.6 +/- 3.4% inhibition). The antibody binds antigen 9 in the LPS; however, this reaction was inhibited by antibodies directed against both this antigen and an adjacent antigen, 12. Anti-12 antibodies presumably inhibit by steric hindrance and their importance in the test is discussed. Thus, the assay potentially detects (only) those systemic infections caused by salmonellae that possess antigen 9 or 12 (viz. S. typhi and S. paratyphi A).