Serum incubation of monoclonal antibodies chelate labeled by DTPA, benzyl-EDTA and benzyl-TETA with 111In, 57Co, and 67Cu demonstrated marked differences in their stability. In serum, 111In-benzyl-EDTA-antibody was more stable than 111In-DTPA-antibody. Cobalt-57 or 67Cu chelated antibody were less stable than either 111In chelated antibody; 67Cu was only firmly attached to the antibody as 67Cu-benzyl-TETA-antibody. The relative stability of the radiometal chelated antibodies was paralleled by the relative stability in serum of the radiometal chelates themselves. These in vitro studies suggest that in vivo behavior of metal chelates exposed to a complex protein environment cannot be predicted by classical equilibrium constants.