The biosynthesis of prostaglandins by human inflamed skin was studied in 8 patients with primary irritant dermatitis induced by benzalconium-chloride. Inflamed skin from these patients evidenced an increased ability to synthesize prostaglandins. In the presence of exogenous arachidonic acid in the incubation medium, the activity formed was about 80% greater in inflamed skin than in non-inflamed control skin. In prostaglandin E1-equivalents the concentration amounted to 8.09 +/- 1.59 ng/mg protein nitrogen and 4.51 +/- 1.24 ng/mg protein nitrogen, respectively (mean values +/- S.E.). When inflamed skin was incubated without excess of exogenous precursor acids in the incubation medium the activity formed was about 80 times lower and the values were similar to those of non-inflamed skin. Thus, the present results support the view that the prostaglandin system is activated in primary irritant dermatitis, thereby providing a basis for future therapeutic attempts to control this disorder.