WIN 66306 (1a), a cyclic peptide containing a novel amino acid, was isolated as a neurokinin antagonist from an Aspergillus species, labelled SC230. Conditions that maximized the production of 1a were developed, leading also to production of the related compound WIN 68577 (2) and rosellichalasin (3). Both 2 and 3 were more active in the rat NK1 than in the human NK1 receptor binding assay, while 1a was more active at the human receptor with an inhibitor affinity constant of 7 microM.