With addition of methyl oleate, the increased yield of antibiotic production by nigericin-producing Streptomyces hygroscopicus NRRL B-1865 also resulted in the isolation of three additional polyether antibiotics. Two of these are abierixin and epinigericin, as new antibiotics. The third antibiotic is grisorixin. The production of both abierixin (opened ring A and 30-CH₂OH) and grisorixin (ring A and 30-CH₃) poses the problem of the identity of the last pathway precursor of the major metabolite, nigericin (ring A and 30-CH₂OH). Transformation experiments of abierixin by S. hygroscopicus gave negative results. Hydroxylation of grisorixin to nigericin by S. hygroscopicus represents the final step in nigericin biosynthesis.