A fiber-optic fluoroimmunoassay system for house dust mite allergen (Der f1) is proposed. We optimized or selected optical components for a system, such as optical filters, fluorescence detectors, and optical fiber probes, to detect fluorescence of a cyanine dye (Cy5). We constructed the excitation-emission system for optimized conditions and measured fluorescence intensitites of dilute solutions of Cy5. The measurement range was from 100 to 105 nmol/l. Next we applied the optical system to fluoroimmunoassays for Der f1. Anti-Der f1 capture antibodies were immobilized onto the end surface of a probe by physical adsorption. Diluted Der f1 solutions and Cy5-labeled antibodies were incubated with capture antibodies to form immune compexes. After all immune reactions were finished, fluorescence from antibodies was measured using the system. Der f1 was quantified in the range of 31.25 to 500 ng/ml. This study suggests that a fluoroimmunoassay system is a base technology to construct a monitoring device for enviromental allergens.