The topical application to human skin of a 1:1 aqueous solution of ammonium hydroxide in a well drilled into a plastic block will provoke an intra-epidermal blister in an average time of about 13 min. The blister roof can be used for physico-chemical analyses of the horny layer while the base is suitable for studies of wound healing, bacterial infections, etc. The minimal blistering time (MBT) increases directly as the number of cell layers of the stratum corneum and ranges from 3 to 57 min in different regions and persons. The intensity of the dermatitis provoked by a 24 h exposure to sodium lauryl sulphate is strongly correlated with the MBT. The latter is therefore a reliable measure of cutaneous irritability. The blisters are virtually painless. The inflammatory reaction clinically and histologically is slight. Healing is rapid without scarring.