Leaflet abscission in Sambucus nigra is precipitated by cell wall degradation which is restricted to the site of cell separation. Accompanying wall breakdown is an increase in the activity of the enzyme polygalacturonase (PG) (E.C. 220.127.116.11) and this rise is primarily confined to the abscission zone tissue. The polygalacturonase has a pH optimum of 4·4 and has the characteristics of an endo-acting enzyme. The elevation in enzyme activity is the result of an increase in at least two isoforms of PG as revealed by polyacrylamide gel electrophoresis of the natured protein. Leaflet abscission in S. nigra is associated with an increase in the solubility and depolymerization of polyuronides from the cell wall. These observations are discussed in relation to the mechanism of cell separation during ethylene-stimulated leaf abscission.