An immunotoxin (IT) was prepared from monoclonal antibody (MoAb) 115D8 and ricin A chain. MoAb 115D8 is directed against the carcinoma-associated sialomucin MAM-6. In a protein synthesis inhibition assay this IT was cytotoxic for the human breast cancer cell line T47D. Using postembedding immunoelectron microscopy the binding and intracellular routing of the IT in T47D cells were studied by simultaneous labeling of both IT moieties, MoAb and A chain, and compared with the fate of native ricin and MoAb 115D8. The IT was internalized into the cell by two different pathways: one via coated pits-coated vesicles followed by transport to the lysosomes and one via large enclosed invaginations of the plasma membrane which apparently fused with lysosomes. This internalization was similar to the endocytosis of MoAb 115D8. During transport via both pathways the IT remained intact until it reached the lysosomes as suggested by the observation that the labels for 115D8 and ricin A chain remained closely associated. Moreover, in areas with abundant endocytic vesicles the labels for both IT moieties were also found in the cytosol, suggesting that intact IT is translocated from the vesicles into the cytosol. In control experiments, native ricin, but not unconjugated MoAb 115D8, was found in the cytosol after internalization. Data presented here show for the first time the complete intracellular pathway of an antibody-ricin A chain conjugate, including the translocation of the A toxin subunit into the cytosol. This IT may be useful for therapy of those tumors which express a high level of MAM-6 on the cell surface.