The specificity of six peroxidase-labelled anti-albumin monoclonal antibodies was studied by allowing them to react with albumin-derived fragments of known structure comprising the whole albumin molecule. The epitopes corresponding to these antibodies were located on defined regions of the albumin molecule extending from the N to the C-terminal ends. These antibodies have been used for investigating by ELISA the conformational alterations of albumin molecule brought about by the N-B transition in twelve individual sera. The results do not show any significant differences between the sera. The N-B transition involves essentially the N-terminal portion of the albumin molecule. There also exists as modification of the C-terminal region which is however much less pronounced than the one in the N-terminal region. In addition, it is not observed when using isolated albumin.