Ester derivatives of 5-O-mycaminosyl tylonolide (OMT) of the formula ##STR1## wherein R and R.sup.1 are selected from hydrogen, optionally substituted C.sub.1 -C.sub.5 -alkanoyl or optionally substituted benzoyl, phenylacetyl, or phenylpropionyl; R.sup.2 is hydrogen or an acyl group selected from: ##STR2## p is 0 or 1; m and n are integers from 0 to 4; R.sup.3 is hydrogen, halo, C.sub.1 -C.sub.4 -alkyl, C.sub.3 -C.sub.8 -cycloalkyl, phenyl, C.sub.5 -C.sub.8 -cycloalkenyl, naphthyl, indenyl, tetralinyl, decalinyl, adamantyl, cinnoxacinyl, a monocyclic heterocyclic ring system comprising 3 to 8 atoms in the ring or a bicyclic heterocylic ring system comprising 6 to 11 atoms, provided that at least 1 atom of the ring system is carbon and at least 1 atom of the ring system is a heteroatom selected from O, N, and S; and wherein R.sup.3 and the connecting alkyl groups--(CH.sub.2).sub.m -- and --(CH.sub.2).sub.n -- are optionally substituted by one or two halo, methyl, ethyl, methoxy, amino, N-protected-amino, methylamino, dimethylamino, nitro, acetoxy, acetamido, azido, carbomethoxy, carboxamido, cyano, or hydroxyl groups, provided that, if the substituent is other than halo or alkyl, there can be no more than one substituent on any connecting --CH.sub.2 -- group; X is O, S, --NH--, --N(CH.sub.3)--, --C.tbd.C--, --CH.dbd.CH--, --C(CH.sub.3).dbd.CH--, --CH.dbd.C(CH.sub.3)-- or --C(CH.sub.3).dbd.C (CH.sub.3)--; R.sup.4 and R.sup.5 are C.sub.1 -C.sub.5 -alkyl or optionally substituted phenyl or benzyl; provided that at least one of R, R.sup.1 and R.sup.2 must be other than hydrogen and that, when R.sup.1 is other than hydrogen, R must also be other than hydrogen; and salts thereof; which are useful antibiotics or intermediates to antibiotics, are provided.