Gel-filtration analysis of a mixture of cyclodextrin (CyD) and lankacidin C showed that fl-CyD had strong, ?'-CyD weak and a -C y D no affinity for lankacidin C. Lankacidin C production activity, which was assayed by measuring the incorporation of L-[methyl-14C-]methionine into the lankacidin molecule, was the greatest with cells grown in the presence offl-CyD, less with y-CyD and the least with a-CyD. Lankamycin and T-2636M, which are by-products in lankacidin C fermentation, were not included by fl-CyD and their production was not stimulated by fl-CyD. It was apparent that the stimulatory effect of C y D was closely related to the formation of an inclusion complex between C y D and the antibiotic. Lankacidin C biosynthesis was repressed by preincubating cells with lankacidin C, while the repressive effect of lankacidin C was abrogated by the inclusion by fl-CyD. Thus, abrogation of feed-back repression seems to be a main mechanism of the effect of CyD. However, a -C y D , which had no affinity for lankacidin C, stimulated the production to the least extent and exhibited a complementary effect on the stimulation by fl-CyD or y-CyD, a -C y D also caused a change in cell morphology and cell-surface hydrophobicity. It was assumed that the modification of the cell surface is a secondary mechanism of the effect of CyD.