An enzyme-linked immunosorbent assay (ELISA) was developed to measure concentrations of the specific F1 antigen of the plague bacillus in biological fluids. The assay employed a monoclonal antibody to capture the antigen. Sensitivity of the assay was 0.4 ng of F1 antigen. ELISA-inhibition was used to confirm the specificity of the reactions.This assay detected F1 antigen in two of ten sera from patients with acute bubonic plague and indicated that antigenaemia in man during plague may reach levels of 4-8 mug of F1 antigen per ml of serum.The probability for a correct serodiagnosis of plague was improved when the patients' sera were tested for both antibody and antigen. Two patients with antigenaemia did not have antibody, while two patients with antibody lacked antigenaemia.