Products of lipoxygenase metabolism are known to play a role in the pathogenesis of psoriasis. Six bisbenzylisoquinoline (BBIQ) alkaloids, oxyacanthine, armoline, baluchistine, berbamine, obamegine, aquifoline, isolated from Mahonia aquifolium, were tested for lipoxygenase inhibition. Berbamine and oxyacanthine were the most potent lipoxygenase inhibitors, whereas aromoline and baluchistine exhibited only very low potencies. Oxyacanthine and berbamine were also among the most active compounds to inhibit lipid peroxidation. Between the results of lipoxygenase inhibition and the lipid peroxidation a linear correlation was found (r = 0.9533). Our data suggest that in the mechanism of lipoxygenase inhibition by these alkaloids, inhibition of lipid peroxide substrate accumulation, either by direct reaction with peroxide or by scavenging or lipid-derived radicals, may play a role. Inhibition of lipoxygenase by these compounds may contribute to the therapeutic effect of Mahonia aquifolium extracts in treatment of diseases in pathogenesis of which he products of lipoxygenase metabolism are involved.