Abstract: An improved one-step immunochromatographic assay which involves the binding of a predetermined amount of analyte to an antibody, enabling the control of the assay sensitivity, is disclosed. The system is especially useful as a controlled sensitivity fecal occult blood assay. Antibodies to a desired analyte, present at a predetermined concentration, are deposited on the sample pad of the reaction unit. The antibody binds analyte present in the sample, up to a threshold amount. Analyte which is present in the sample at a level above the threshold amount proceeds unbound through the sample pad and onto a membrane, where it reacts with an antibody-coated latex and a second, immobilized antibody to generate a positive signal.