The class II major histocompatibility antigens at the cell surface exist as heterodimers of alpha and beta subunits. During biosynthesis, these subunits are associated with a third chain, the invariant (I) or I chain. Association with the I chain occurs early in biosynthesis in the rough endoplasmic reticulum and persists during transport through the Golgi apparatus. One of the two alpha subunit N-linked oligosaccharides and the single beta subunit N-linked oligosaccharide are converted to the complex form during Golgi transit. In the human system, both I chain N-linked oligosaccharides can also be processed to the complex form, and at least two O-linked oligosaccharides can be added to the I chain. At some point during transit to the cell surface, class II antigens associate with a proteoglycan bearing chondroitin sulfate side chains. Complexes containing alpha, beta and I chain subunits and the associated proteoglycan accumulate in human B-cell lines treated with the ionophore monensin, an inhibitor of Golgi transport, suggesting that this may be a biosynthetic intermediate in class II antigen transport and assembly. Prior to cell surface expression of class II antigens, the exocytic pathway which they follow intersects the endocytic route, followed by certain ligands internalized by receptor-mediated endocytosis. The I chain appears to dissociate from mature class II alpha, beta dimers prior to their cell surface expression but following the intersection of the exocytic and endocytic pathways.