We describe immunoassay techniques that take advantage of the novel properties of phycobiliprotein fluorescent dyes. These dyes, which can be isolated from a wide variety of algae, exhibit extremely high absorptivities, high quantum efficiencies, and excitation and emission bands across the visible spectrum. These stable, hydrophilic proteins can easily be linked to antibodies by conventional protein cross-linking reagents. One assay technique presented here demonstrates the use of phycobiliproteins in a solid-phase "sandwich" assay, which achieves picomolar sensitivity. In the other technique the unique spectral properties of phycobiliproteins are applied to fluorescence excitation transfer immunoassay.