A pesticide series of 36 substances was patch tested in 652 subjects to establish the optimal test concentration, and the frequency of irritant and allergic reactions. Allergic reactions to fungicides were found in 46: thiophthalimides (captan, folpet and difolatan) were the most common. Irritant and allergic reactions to other pesticides (insecticides, herbicides, etc.) were rare. Sensitivity to pesticides was significant in patients who work, or have worked on the land. Cross sensitivity to bisdithiocarbamates or thiophthalimides is possible.