A fiber optic biosensor was used to detect the fraction 1 (F1) antigen from Yersinia pestis, the etiologic agent of plague. The instrument employs an argon ion laser (514 nm) to launch light into a long-clad fiber and measures the fluorescence produced by an immunofluorescent complex formed in the evanescent wave region. This sensing area is a short section (12.5 cm) at the end of the optical fiber from which the cladding has been removed and in which the silica core has been tapered. Capture antibodies, which bind to F1 antigen, were immobilized on the core surface to form the basis of the sandwich fluoroimmunoassay. The ability to detect bound F1 antigen was provided by adding tetramethylrhodamine-labeled anti-plaque antibody to form fluorescent complexes. The evanescent wave has a limited penetration depth (< 1 lambda), which restricts detection of the fluorescent complexes bound to the fiber's surface. The direct correlation between the F1 antigen concentration and the signal provided an effective method for sample quantitation. This method achieved a high level of accuracy for determining F1 antigen concentrations from 50 to 400 ng/ml in phosphate-buffered saline, serum, plasma, and whole blood, with a 5-ng/ml limit of detection. Subsequent blind studies, which included serum samples from patients, yielded results in good agreement with measurements by enzyme-linked immunosorbent assay. A major advantage of the fiber optic biosensor is that results can be generated within minutes while isolating the user from hazardous samples. These factors favor development of this biosensor into a facile and rapid diagnostic device.