The major C14-labeled substance in sieve tube exudate of M. pyrifera is D-mannitol, comprising 3.6% (w/v). No sugars are detectable. Certain amino acids also possess some C14-labeling and occur in significantly high concentrations in exudate. The exudate contains negligible ether-soluble lipid, but has a large amount of protein and a high concentration of K+ Neither protein nor lipid become labeled significantly in sieve tubes during short-term translocation experiments with C14. In general the chemical composition of the assimilate stream is comparable to that of vascular plants and does not, consequently, necessitate a different mechanism for translocation.