Abstract: A dye-providing composition is useful in various diagnostic assays wherein a peroxidase-labeled specific binding species is used. This composition is substantially free of peroxidase and such labeled species, and comprises an imidazole leuco dye and 4'-hydroxyacetanilide present in an amount up to about 2.5 mmolar. This composition can be included as part of a diagnostic test kit.

Abstract: A method is disclosed for conducting an assay for an analyte. The method comprises causing a specific binding pair member in a first aqueous medium to become bound to a first bibulous member by contacting a portion of the first bibulous member with the medium. The first bibulous member is in liquid receiving relationship with an absorbent member. The contacting is carried out under conditions wherein the first medium traverses the first bibulous member and at least a portion of the absorbent member by capillary action. The method further comprises causing the first bibulous member to come into liquid receiving relationship with a second bibulous member. A reagent in a second aqueous medium is absorbed by and preferably becomes a non-diffusively bound to the second bibulous member in relation to the presence of analyte in the first medium.