Abstract: A method was described for labelling CEA monoelonal antibody (CEA-McAb) with the a-emitting nuclide ~(211)At via diazo salts of p-phenylenediamine or benzidine. Aromatic diamines were transformed into diazo salts and subsequently both ~(211)At and CEA-McAb react with diazo salts to produce ~(211)At-CEA-McAb conjugates. The reaction and purification required about 2.5 h. Sephadex G-75 column was used to separate the labelled CEA-McAb from reactive products and the labelling yield was at least 30% of the initial activity of ~(211)At. The specific activity of ~(211)At-CEA-McAb (2.2—3.7)×10~4Bq/ug(McAb) could be achieved. The results of tissue distribution of ~(211)At in mice showed that ~(211)At-CEA-McAb conjugates were stable in vivo.