The development of the monoclonal antibody technique¹ has led to renewed interest in whether the growth of tumour cells may be specifically inhibited by antibodies or antibody–toxin conjugates directed against antigenic determinants selectively expressed on tumour cells^2–10. We have recently obtained monoclonal antibodies against the transferrin receptor of human cells^11,12, which is thought to have an essential role in transport of Fe across the cell membrane and which is selectively expressed on proliferating cells in vivo and in vitro^13–17. In some cases, transferrin receptors can be used as a marker to distinguish between tumour cells and normal tissue^11,15–17. Here we show that human tumour cells are specifically killed in vitro by anti-transferrin receptor antibody covalently coupled to ricin or diphtheria toxic subunits. In experiments designed to test the effectiveness of these antibody–toxin conjugates in vivo, we found that anti-transferrin receptor antibody alone inhibits the growth of human melanoma cells in nude mice.