Schistosoma mansoni cercarial transformation and early stages of penetration can be correlated to specific eicosanoid products. Cercarial eicosanoids are suggested to play immunoregulatory roles during penetration. We examined production of cercarial eicosanoids by S. mansoni compared with that of the duck parasite Trichobilharzia ocellata after incubation with linoleate. Secretions were separated by reversed-phase high pressure liquid chromatography and also quantified by radioimmunoassay. The 2 parasites, differing in their immune evasion, synthesized the same types of prostaglandins, leukotrienes, and hydroxy-eicosatetranoic acids in similar quantities. Eicosanoid fractions of both species also showed a similar inhibition of superoxide production by human neutrophils. The coincidence of eicosanoid production suggests a function of eicosanoids in processes that are similar in both species.