Biliproteins were extracted from representatives of the Cyanophyta, Rhodophyta, and Cryptophyta and purified. Both purified and crude biliproteins were used to stimulate rabbit antibody directed specifically against the biliproteins. The antigenic and immunogenic inter-relationships of these proteins were investigated by the Ouchterlony double diffusion technique. C-phycocyanins from all sources were found to be antigenically and immunogenically related and apparently also related to allophycocyanin but not to any of the phycoerythrins. Larger antigenic differences among phycoerythrins from different groups of algae were discovered. The role of aggregation of the individual biliproteins in their immunochemistry was characterized. Attempts were made to determine the phylogenetic significance of these results. The immunochemical aspects of the biliproteins were striking in that protein antigens from vastly different cell types were found to be closely related. This relationship may be interpreted as supporting the suggestion that Rhodophyta evolved from Cyanophyta or from some common ancestral stock.