A preparation of purified bovine IgG1 was found to contain 0.5 moles of Cu2+ ions per mole of protein. Scatchard analysis of the binding of Cu2+ ions to bovine IgG1 indicated the presence of 4 +/- 1 binding sites with an affinity of 2 x 10(5) M-1. Calculations indicate that in bovine plasma, IgG1 will not compete significantly with albumin for Cu2, whereas a proportion of Cu2+ may well be bound to IgG1 in colostrum. Physiological levels of Cu2+ were found to enhance the interaction between IgG1 antibody and Brucella abortus in the presence of albumin. Irrespective of whether IgG1 binds Cu2+ in vivo, it is possible that Cu2+ can influence the results of serological tests.