Phloem exudates collected from the secondary phloem of Tilia platyphyllos, from the flower stalks of Yucca flaccida and from stems of Salix triandra (obtained by the aphid method) were incubated with (32)PO4 (---). The radioactivity was incorporated into a series of organic compounds very similar to those synthesized after the whole phloem is incubated with (32)PO4 (---). This synthetic ability of the exudate was completely destroyed by heating; therefore it is very probably due to enzymes. 2. ATP is one of the very first compounds labeled after incubation of the sieve tube sap with (32)PO4 (---). It already shows radioactivity after 10 sec. 3. The phloem exudate of Yucca flaccida does not lose the synthetic ability even after several hours of secretion. This indicates that the enzymes and the substrates for the phosphorylations are secreted continuously into the lumen of the sieve tubes. 4. The formation of the organic phosphorus compounds in the sieve tube sap is in accordance with the results of the enzyme analyses in this material (Kennecke et al., 1971).
