WIN 64821 (1) is a substance P (SP) antagonist isolated from a fungal culture (Aspergillus sp., SC319). It is a symmetrical dimer biosynthesized from four aromatic amino acid molecules: each equivalent half of the dimer is constructed from one molecule of phenylalanine (Phe) and one molecule of tryptophan (Trp). Feeding analogs of Phe, Trp, and other amino acids to intact cells of SC319 has yielded 36 biosynthetic analogs of WIN 64821. The analogs fall into three categories: substitutions on the indoline ring, substitutions on the Phe-derived phenyl ring, and replacement of the phenyl ring by an aliphatic group. In addition, these directed biosynthesis experiments generated asymmetrical dimers (derived from three amino acids) and, often, symmetrical dimers (derived from two amino acids). The relative SP binding affinities of several analogs suggest involvement of both the indoline and phenyl moieties in SP receptor binding.