Protein subunits present in phloem exudate from 17 cultivars, 5 species and 3 genera of the Cucurbitaceae have been fractionated by SDS-gel electrophoresis. The degree of difference in the phloem protein patterns appears to reflect the taxonomic relationships of the plants: there were major differences among genera, significant differences and similarities among species, and relatively few differences among cultivars of a single species. A representative of another family (Acer pseudoplatanus; Aceraceae) provided a completely different electrophoretic pattern. Each plant displayed a consistent protein pattern, irrespective of the organ from which exudate was obtained or the age and physiological status of the plant. Neither complete etiolation nor transition to the flowering phase effected any change in phloem proteins. The differences in phloem proteins between plants and the unusual properties of major subunits from different protein complements, render improbable many of the present ideas concerning the origin and function of P-protein.