An inflammatory response of the skin to dithranol-induced free radicals seems to be essential for its clinical efficacy. In normal volunteers this response was evaluated at the level of the microvasculature following 30 min, 2 h and 24 h applications, using a functional parameter (erythema) and a biochemical parameter (alkaline phosphatase). The results of 'short contact' and 24 h applications were similar. In all schedules a maximum erythema was seen 2-3 days after the application which had resolved totally after 6-8 days. A marked discrepancy was established between the duration of functional and biochemical abnormalities; the alkaline phosphatase activity reached a maximum 1 day after the culmination of the erythema and persisted up to at least 7 days after disappearance of the erythema. These findings are discussed in the light of the day-to-day management of psoriasis with dithranol.