Two new cyclopropyl and lactone containing eicosanoids, constanolactone A and B, were isolated as synthetic diacetate derivatives from the temperate red alga Constantinea simplex and their structures determined by spectroscopic means. A hydrolysis product of one of these helped to define the lactone size in these new eicosanoids. The co-occurrence of three known 12-lipoxygenase metabolites, 12-S-HETE, 12-S-HEPE, and 12-oxododeca-5(Z), 8(E), 10(E)-trienoic acid, supports a 12-1ipoxygenase origin for the new compounds. © 1990.