Washing symbiotic Chlorella algae freshly isolated from green hydra with 0.05% sodium dodecyl sulphate was shown to remove virtually all contaminating host material, previously a severe constraint in quantifying movement of metabolites from host to symbionts. When brine shrimp labelled with 3H-leucine were fed to hydra in symbiosis either with the native strain of Chlorella (E/E hydra) or two strains cultured from Paramecium bursaria (E/3N and E/NC hydra), it was found that after 24 h 3-4% of the total radioactivity retained by the symbiosis was present in the algae. Analysis of the free amino acid pool of symbiotic algae from E/E hydra showed that over 70% of the radioactivity was associated with leucine, and significant amounts of radioactivity were retained by these algae for at least five days following a single feeding with radioactive brine shrimp. In both E/E and E/NC hydra, the amount of radioactivity per unit protein was considerably less in the symbionts than in the host, suggesting that access to host amino acid pools were limited. These results are discussed in terms of the possible role and regulatory significance of amino acids as a nitrogen source to symbiotic Chlorella, and of the cost to the host in maintaining the symbiosis.