A passive hemagglutination (PHA) test for serodiagnosis of enteric fever was developed by sensitizing glutaraldehyde-preserved erythrocytes with lipopolysaccharide from Salmonella serogroups A, B, C, and D singly or simultaneously. The lipopolysaccharide-sensitized erythrocytes were tested with sera from 200 blood donors, 100 patients whose hemoculture was positive for Salmonella species, and 10 patients septicemic for other members of the family Enterobacteriaceae. The PHA test was positive in 90% of 28 acute-phase serum samples from patients with enteric fever from one hospital and in 93% of 72 acute-phase serum samples from another hospital. It was also positive in 100 and 60% of early- and late-convalescent-phase sera, respectively. The PHA test was negative in all patients septicemic for other members of the Enterobacteriaceae. Absorption of sera from patients with enteric fever with lipopolysaccharide from other members of the Enterobacteriaceae did not reduce PHA titers, indicating the specificity of the PHA test. Simultaneous sensitization with lipopolysaccharide from Salmonella serogroups A, B, C, and D was useful as a screening test in a limited trial with 28 acute-phase sera, 10 early-convalescent-phase sera, and 17 late-convalescent-phase sera. The PHA test is indeed a simple, sensitive, specific, and rapid test supplementing hemoculture in laboratory diagnosis of enteric fever.