Seco compounds from a class of tylosins represented by the formula I whereinR stands for H or CH.sub.3,R.sup.1 stands for H, CH.sub.3, C.sub.1 -C.sub.3 acyl or aryl sulfonyl,R.sup.2 stands for H and R.sup.3 stands for NH.sub.2 or OH, or R.sup.2 and R.sup.3 together stand for .dbd.O or .dbd.NOH,R.sup.4 stands for H or C.sub.1 -C.sub.3 acyl,and the line - - - stands for a double or a single bond,and to a process for their preparation are provided.Oximation of 4'-demicarosyl-8a-aza-8a-homorelymycin causes breaking of the lactam to obtain a seco compound. The seco compound can be subjected to reductive N-alkylation or to conversion of the hydroxyimino group into a keto group and then optionally to N- or N,O-acylation, a catalytical hydrogenation of the double bond or a reduction of the ketone or a reduction of the hydroxyimino group into an amino group.