T o determine whether lipoxygenase products of arachidonic acid metabolism are released in vivo during human allergic cutaneous reactions, we serially assayed chamber fluid placed over denuded skin sites for the presence of both C-6 peptide leukotrienes (e.g., LTC4, LTD4, and LTE4) and leukotriene B4 (LTB4), using radioimmune assay and HPLC separation, and compared it to histamine (assayed radioenzymatically) in 13 atopic and two nonatopic volunteers. Skin chamber sites challenged with ragweed or grass pollen antigen (250-750 protein nitrogen units/ml) for the first hour and phosphate-buffered saline (PBS) for the next 3 h were assayed hourly and compared to sites challenged with PBS alone. As assessed by HPLC, LTC4 composed greater than 85% of the C-6 peptide leukotriene released at any skin site, whereas little LTD4 or LTE4 was detected. LTC4 was present in significantly greater concentrations at antigen sites as compared to PBS-challenged sites throughout the 4-h period. Minimal concentrations of LTB4 were found throughout this time period and were not different at antigen or PBS sites. Histamine was present in significantly greater concentrations at antigen rather than PBS sites, but the pattern of release was different from that of LTC4. Peak histamine release invariably occurred during the first hour and decreased progressively thereafter, whereas the greatest amounts of LTC4 were detected during the 2nd to 4th hours. The amount of LTC4 accumulating at the site was dependent upon the dosage of antigen used in the epicutaneous challenge. We have demonstrated in this study that of the leukotrienes assessed LTC4 is released in the greatest quantity in situ during in vivo allergic cutaneous reactions and that it is present at such sites for at least 4 h after antigen challenge. Since intradermal injection of LTC4 in humans induces wheal and flare responses that persist for hours, our findings support the hypothesis that LTC4 is an important mediator of human allergic skin reactions.