Action of Asc, a single dominant Mendelian gene controlling disease response at the whole plant level, was detected at the level of individual cells. Protoplasts, freshly isolated from resistant (Asc/Asc) and susceptible (asc/asc) tomato isolines, were differentially sensitive to AAL toxin as observed in planta. Protoplast mortality was toxin concentration dependent in both isolines and required the same 24-36 h time frame as necrosis in leaflet tissue, but asc/asc protoplasts were 10-fold less sensitive (0.15 µM) and Asc/Asc protoplasts were over 100-fold less sensitive (> 150 µM) than corresponding leaflet tissue. Light was required for necrosis in leaflets but not in protoplasts; asc/asc leaflets kept in darkness remained green for 48 h at 100-fold excess toxin concentration whereas protoplasts showed equivalent sensitivity in light or dark. Aging diminished toxin sensitivity in both leaflets and in protoplasts. Fully expanded asc/asc leaflets were less than half as sensitive to AAL toxin (~ 30% necrosis) as rapidly expanding leaflets (~ 87% necrosis) while protoplasts from equivalent leaflets were only 80% as sensitive to AAL toxin 2 days after isolation and insensitive after 9 days. These data indicated that the action of the Asc gene is expressed in protoplasts, the process of cell death is analogous in single cells and intact leaves, but that additional processes expressed in differentiated tissue can modify Asc response to AAL-toxin.