Tetracycline analogs were designed and synthesized and their bone affinities were tested on hydroxyapatite. The results showed that the carbonyl-amide-enol structure in A ring and phenol-ketone structure in BCD ring may be responsible for tetracycline's high bone affinity and either A ring or BCD ring has a planar conformation is essential. 2007 Ling Ling Weng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.