Both irritation and staining reaction of anthralin on the skin are the two most important problem of therapy. Irritation, such as erythema, edema and staining by p. lg anthralin ointment were studied with the chamber-testing technique in 21 psoriatic patients. We campared the skin reaction of short exposure time, such as 1 hour, 2 hours, and 3 hours with those of exposure of 24 hours through 3 days after application. The results were as follows: l. Incidence of erytherna reaction was 81.9% in exposure of 1 hour, 85.7%, in 2 hours, 90.5%, in 3 hours and 100% in exposure of 24 hours. Degree of erythema reaction was increased according to duration of application. 2. There were no edematous reactions in exposure of 1 hour or 2 hours, 9,5% in 3 hours and 28,6% of grade 1 reaction in 24 hours. 3. There was no staining reaction in exposure of 1 hour, 48% in 2 hours, 9.5%, in 3 hours and 71.4% in 24 hours. Degree of staining reaction elicited by short exposure time were all weaker than reaction caused by 24 hours exposure.