Addition of pure methyl oleate to aStreptomyces hygroscopicusNRRL B-1865 culture improved polyether antibiotic production, particularly of the closely related nigericin and abierixin, more than did technical methyl oleate. Among the fatty acid constituents of technical methyl oleate, the methyl esters of stearate and particularly linoleate were detrimental to growth and antibiotic production, while the methyl esters of palmitate and palmitoleate had the same effect as technical methyl oleate. The presence of Tween 80 or cyclodextrin inS.hygroscopicuscultures supplemented with methyl oleate increased valine uptake and antibiotic production. Antibiotic production was highest with a mixture of 1% methyl oleate, 0.1% Tween 80, and 0.1% cyclodextrin. The results suggested that the stimulatory effect of methyl oleate might be enhanced by a substance increasing the accumulation of branched amino acids known to be direct precursors of polyether antibiotics. The rate of abierixin synthesis was more important than that of nigericin at the end of the culture. The nigericin/abierixin ratio decreased from 6 in basal medium supplemented with methyl oleate to 2 in basal medium supplemented with methyl oleate, Tween 80, and cyclodextrin. A strong decrease in culture pH seen in the presence of Tween 80 and cyclodextrin might lead to a Michael transformation of nigericin to abierixin.Key words:Streptomyces hygroscopicus, nigericin, abierixin, methyl oleate, Tween 80, cyclodextrin.