Tetracyclines and their metabolites in egg white and yolk

4-Epimers of tetracycline (TC) and chlortetracycline (CTC) were formed in v ivo in laying hens and excreted in substantial amounts besides the residues of the administered parent compounds via the egg. The recent observation o f isochlortetracycline (i-CTC) and its LC-epimer as being the principal met abolites in eggs after feeding CTC could be confirmed. The ratio of 4-epime r to parent compound of TC, CTC and i-CTC was lower in egg white and higher in yolk and did not change during medication with TC or CTC. Further, yet unidentified, in vivo formed metabolites were observed.