A detached-leaf bioassay was developed to quantify relatively small differences in resistance expression of tomato. Using either of two inoculation methods, ‘drop’ or ‘spray’, the bioassay indicated that older leaves, and older leaflets of a single leaf, were more susceptible to attack by Phytophthora infestans than younger leaves and leaflets. Although the age of the tissue clearly influenced lesion expansion, the age of the whole plant did not significantly affect the level of resistance.