The biliproteins C-phycocyanin and allophycocyanin were purified from the blue-green alga Anabaena variabilis by ammonium sulphate fractionation and gel filtration. 2. An assay procedure that enabled the proportion of the two pigments, present as a mixture, to be determined was devised by using the data provided by spectrophotometric analysis of the purified biliproteins. 3. The degree of association and relative proportions of the two pigments were analysed by the application of this procedure to the components separated by thin-layer gel filtration. 4. The C-phycocyanin/allophycocyanin ratio remained essentially constant in algal extracts prepared at various stages throughout the growth cycle or after growth under conditions of reduced illumination. 5. The behaviour of the C-phycocyanin aggregate species from Anacystis nidulans suggested that they were of appreciably lower molecular weight than those observed in extracts of Anabaena variabilis.
