Epidermal Langerhans cells are macrophage-like la+ leukocytes that are critically involved in cutaneous immune reactions. Because macrophages exert their immunoregulatory activity in part by generation of oxygenated arachidonic acid metabolites, we systematically studied arachidonic acid transformations by purified guinea pig Langerhans cells and compared them with mixed epidermal cells and Langerhans cell-depleted keratinocytes. Products formed from arachidonic acid by cell homogenates were measured after thin-layer or reverse-phase high-pressure liquid chromatographic separation. In addition, leukotriene B4 and C4 formation was assessed in supernatants of Ca ionophore A23187-challenged intact cells by radioimmunoassay. Mixed epidermal cells converted arachidonic acid predominantly via cyclooxygenase and 12-lipoxygenase pathways. The main products were prostaglandin D2 (PGD2) and 12-hydroxyeicosatetraenoic acid (12-Hete), although significant amounts of PGE2, PGF2 alpha, and 6-keto-PGF1 alpha were formed as well. PGD2 synthesis was dependent on the presence of reduced glutathione. The product spectrum formed by Langerhans cell-depleted keratinocytes was virtually indistinguishable from mixed epidermal cells. In contrast, Langerhans cells showed a markedly different metabolism of arachidonic acid. They exhibited an exceedingly high PGD2-generating capacity, whereas only minor amounts of 12-HETE and very low amounts of other prostaglandins were synthesized. The PGD2/12-HETE ratio was 1.22 for mixed epidermal cells and 4.37 for Langerhans cells. Leukotriene production from exogenous or endogenous arachidonic acid could not be demonstrated by either radioenzymatic or radioimmunologic detection methods. We conclude that guinea pig Langerhans cells transform arachidonic acid predominantly to PGD2, which might mediate significant immunoregulatory, inflammatory, and antitumoral activity in the skin.