We have evaluated the effect of an adriamycin conjugate of monoclonal antibody Nd2 (ADM-Nd2) on the growth rate of SW1990 xenografts grown subcutaneously in athymic nude mice. Intravenous or intraperitoneal administration of radiolabeled Nd2 resulted in a maximum tumor accumulation of approximately 45% of the initial dose/g of tumor 3-7 days after administration. However, administration into the tumor produced retention of 1200%ID/g 1 day after, with 50% of this high value remaining even at 7 days after administration. In contrast, intratumoral administration of a non-specific immunoglobulin showed a lower initial retention and rapid loss of label. Both intravenously and intratumorally administered ADM-Nd2 reduced the growth rate of SW1990 xenografts. While a single intravenous administration arrested growth for about two weeks, a single intratumoral injection prevented any increase in tumor size even 45 days after administration. Xenografts treated with ADM-Nd2 showed degenerative changes at the histological level. Neither Nd2 alone nor Adriamycin alone inhibited growth when administered at the same dose as the conjugate.