Newly synthesized class II HLA antigens being transported to the surface of human B-lymphoblastoid cell lines (B-LCL) interact with transferrin-neuraminidase conjugates internalized by means of receptor-mediated endocytosis. Class II antigens, isolated from [35S]methionine-labeled B-LCL after incubation with the conjugates at 37 degrees C, showed extensive desialylation of associated invariant chain and detectable loss of beta-subunit sialic acid on analysis by two-dimensional gel electrophoresis. An equal amount of unconjugated neuraminidase had no effect, and desialylation of class II antigen components was blocked when access of transferrin-neuraminidase conjugates to the B-LCL transferrin receptors was competitively inhibited by the addition of excess iron-saturated transferrin. The conjugates were shown to cycle through the cells in the same way as unconjugated transferrin, being first internalized and then rapidly secreted in an undegraded form. The data suggest that the exocytic pathway taken by class II antigens intersects the route followed by recycling transferrin receptors and that the interaction occurs prior to the dissociation of the invariant chain from the class II antigen complex. Similar intracellular interactions between class II molecules and foreign proteins internalized by antigen-presenting cells may be important in class II antigen-restricted recognition by helper T lymphocytes.