A non-yellowing mutant of Phaseolus vulgaris L. was used to investigate factors involved in chlorophyll breakdown during foliar senescence. The mutant showed physiological changes similar to those of the normal yellowing type during senescence except that leaf chlorophyll did not decline. Transmission electron microscope studies did not reveal appreciable differences in chloroplast ultrastructure between the two genotypes, suggesting that chloroplast membrane integrity was not the factor preventing chlorophyll degradation in the mutant. However, the lack of plastoglobuli in senescent mutant chloroplasts suggested that the lipid environment may be different from that of senescent normal chloroplasts. Banding patterns of total soluble protein, resolved by sodium dodecyl sulphate-poly aery lamide gel electrophoresis showed few, if any, differences between mature non-senescent normal and mutant leaves; however, bands at 14 kD and 58 kD diminished in senescent normal leaves, but remained in senescent mutant non-yellowing leaves.