Abstract: A-21978C cyclic peptide derivatives of the formula ##STR1## in which R, R.sup.1 and R.sup.2 are, independently, hydrogen, C.sub.4 -C.sub.14 -alkyl, optionally substituted C.sub.2 -C.sub.19 -alkanoyl, C.sub.5 -C.sub.19 -alkenoyl or an amino-protecting group; R.sup.3, R.sup.4 and R.sup.5 are hydrogen or (i) R.sup.3 and R.sup.1 and/or (ii) R.sup.4 and R and/or (iii) R.sup.5 and R.sup.2, taken together, may represent a C.sub.4 -C.sub.14 alkylidene group; provided that (1) at least one of R, R.sup.1 or R.sup.2 must be other than hydrogen or an amino-protecting group, (2) at least one of R.sup.1 or R.sup.2 must be hydrogen or an amino-protecting group, (3) the R, R.sup.1 and R.sup.2 groups must together contain at least four carbon atoms, and (4) when R.sup.1 and R.sup.2 are both selected from hydrogen or an amino-protecting group, R cannot be 8-methyldecanoyl, 10-methyldodecanoyl, 10-methylundecanoyl, the mixed C.sub.10 -alkanoyl group of A-21978C.sub.0 or the specific C.sub.