Cellulase (beta-1,4-glucan-glucanohydrolase EC 18.104.22.168) activity increased during abscission and was localized in the cell separation layer of Phaseolus vulgaris L. cv. Red Kidney (bean), Gossypium hirsutum L. cv. Acala 4-42 (Cotton) and Coleus blumei Benth. Princeton strain (Coleus) abscission zone explants. Cellulase activity was optimum at pH 7, was reduced by one-half after heating to 55 degrees for 10 min, and was associated with the soluble components of the cell. Explants treated with aging retardants (indoleacetic acid, (6)N-benzyladenine, and coumarin), CO(2), actinomycin D or cycloheximide had less cellulase activity than untreated controls. Ethylene increased cellulase activity of aged explants after a 3-hr lag period but had no effect on cellulase activity of freshly excised explants. It was concluded that 1 of the roles of ethylene in abscission is to regulate the production of cellulase which in turn is required for cell separation.