Monoclonal antibodies labeled with radiometals such as copper-67 have applications in radioimmunodiagnosis and radioimmunotherapy. Moi et al. [(1985) Anal. Biochem. 148, 249-253] showed that 6-[p-(bromoacetamido)benzyl]-1,4,8,11-tetraazacyclotetradecane- N,N',N",N"-tetraacetic acid (BAT) is an effective reagent for linking copper to proteins, and antibodies labeled with copper radionuclides are currently undergoing clinical trials. Here we describe improvements in the original synthesis that increase the overall yield of BAT to 23%. We also describe a new assay useful to determine the activity of bifunctional chelating agents with thiol-reactive functional groups.