The contributions of the C-ll methyl and C-13 / C-15 methoxy substituents to the affinity of FK-506 for the immunophilin FKBP-12 were investigated. Subtraction of the C-11 methyl group led to a 400-fold drop in affinity. Removal of the C-13 and C-15 methoxy substituents led to a much smaller drop in affinity.