A new analysis has been made on studies of the influence of imino acid content on the changes of collagen thermal stability (tm). It is shown that, for the interstitial vertebrate collagens, there is a strict regularity in the changes of tm depending on hydroxyproline content. No correlation is observed between tm and proline content. Also, no correlation between tm and hydroxyproline content is observed for invertebrate and basement membrane collagens. On the basis of the reported data, the dependence of tm on hydroxyproline content is considered to be not a correlation between tm and the total content of hydroxyproline, but only as the correlation between tm and the content of hydroxyproline occurring at the third position in the sequence (Gly‐R2‐R3)n. The results agree with the idea that the influence exerted by proline and hydroxyproline on the stabilization of the triple helix of collagen is different. Copyright © 1979 John Wiley & Sons, Inc.