Studies with a conjugate of carboxypeptidase G2 (CPG2) and the F(ab')2 fragment of monoclonal anti-CEA antibody, A5B7, have shown specific localisation in a human colon tumour xenograft, LS174T, growing in nude mice. The conjugate reaches a peak concentration in the tumour within 24 h but enzyme activity in blood remains above a critical value for therapeutic purposes for several days. Here we describe a new monoclonal antibody, SB43, raised against CPG2 which is capable of reducing enzyme activity in blood. In vitro studies demonstrated specific binding of SB43 to CPG2 causing inactivation. Moreover, in the nude mouse model SB43 was also capable of inactivating the enzyme in the circulation within minutes of administration. Radiolabelled native SB43 persisted in blood for several days and appreciable non-specific uptake into the xenograft was also observed. Uptake of SB43 by the tumour, with possible inactivation of CPG2 at this site, could be limited by first coupling the antibody to galactose. This ensured recognition and excretion of SB43 and SB43-enzyme complexes via the liver and their rapid removal from the circulation. Galactosylation had no effect on the ability of SB43 to inactivate the enzyme.