In order to obtain maximum analytical performance from an optical immunosensor which uses whole blood as the sample matrix, rigorous selection of the fluorophore and the associated optical filtration is required. Three fluorophores, allophycocyanin (APC), carboxymethylindocyanine succinimidyl ester (C45) and fluorescein isothiocyanate (FITC), were evaluated in whole blood and serum assays using the fluorescence capillary fill device immunosensor. APC gave the best analytical performance. Whole-blood patient samples were assayed using APC as the label and an excellent correlation was obtained with a commercially available immunoassay.