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Ten cases of typhoid fever occurred between 24 August and 1 September 1986 in the vicinity of Silver Spring, Md. Shrimp salad served in a fast-food restaurant was implicated as the source of infection. Stool cultures were obtained from 104 employees, and serum Vi antibodies were assayed in 97 of the employees. Salmonella typhi was isolated from stool cultures of an 18-year-old asymptomatic female employee, who was a food handler. A high level of Vi antibodies (79.0 micrograms/ml), measured by radioimmunoassay, was found in her serum. She had emigrated from an endemic area at the age of 14 years and had visited that endemic area 2 years previously. The causal relation between the carrier and the 10 cases of typhoid fever was confirmed by a common bacteriophage type, denoted "degraded Vi resembling O," in the S. typhi isolates. This phage type is rare in the western hemisphere but common in the endemic area from which the carrier had emigrated. The high level of Vi antibody in the asymptomatic carrier, in contrast to the lower levels in the convalescent- and postimmunization-phase sera, facilitated the identification of the source infection in this outbreak. This radioimmunoassay offers a rapid and standardized method for identifying carriers of S. typhi.