AbstractWe are currently investigating the morphologic aspects of two areas of the basic cell biology of cancer: tumor‐specific surface antigens as targets for immunotoxins, and the phenomenon of multidrug resistance in chemotherapy of human tumors. Colloidal gold cytochemistry has provided a useful method for the electron‐microscopic cytochemical detection of materials endocytosed by cells in culture. This technique has been used to study the internalization pathway of ligands bound to the surface of cancer cells, particularly antibodies for use as immunologic targeting reagents for the construction of immunotoxins. These colloidal gold conjugates with monoclonal antibodies have demonstrated the internalization of these immunologic reagents through coated pits and reeeptosomes, which is a necessary step in the delivery of immunotoxins into the cell where they can mediate their cell‐killing functions. Morphologic methods have been employed for the screening and selection of monoclonal antibodies reactive with the surface of human ovarian cancer cells for use as immunotoxins and have demonstrated the in vivo activity of immunotoxins made with these antibodies andPseudomonasexotoxin in a nude mouse model system. In other studies, we have employed such reagents for the immunocytochemical detection of the surface expression of P170, the cell‐surface efflux pump protein responsible for the phenotype of multidrug resistance in tumor cells, and to investigate the distribution of this protein by using immunocytochemistry in normal human tissues. These results have suggested a role for P170 in normal cell membrane transport of metabolites in various organ