The pH-dependent conformational changes in immunoglobulin M were studied by differential spectrophotometry. It was found that the state of chromophores (tryptophan and tyrosine) which reflects conformational changes of the structure alters stepwise in the course of acidification. The native structure is not restored by neutralization. The recovery of the native structure was obtained only at pH approximately 6.5 of the IgM solution. A possible explanation of concrete conformational transitions during the pH change is proposed. These changes were shown to be similar for IgM and IgG.