The distribution in athymic nude mice of radiolabelled monoclonal antibody 791T/36 has been assessed during continuous infusion from subcutaneously implanted Alzet Osmotic Minipumps. During prolonged administration (up to 15 days) blood levels continued to rise. At 15 days, distribution of radiolabel was virtually identical to that seen after a single parenteral dose. Blood levels were in good agreement to those expected from whole body levels indicating satisfactory entry of antibody into the vascular compartment. Gel filtration chromatography of osmotic minipump contents and circulating radiolabel showed that the antibody had retained its structural integrity. In mice with human tumour xenografts examined a 5-day infusion of a mixture of 131I-791T/36 antibody and 125I-control IgG2b, blood levels of both radiolabels were comparable to those expected from whole body levels and there was effective tumour localization of the antibody to 2.5 times that of control IgG. These studies have demonstrated that prolonged administration of monoclonal antibody is feasible, that antibody enters the vascular compartment satisfactorily and that it can then localize in tumour deposits.