We compared the processing and presentation of the model Ag, hen-egg white lysozyme (HEL) expressed in C3.F6 APC as a fusion protein to three different acid hydrolases: cathepsin D, to an unglycosylated form of cathepsin D, and to pepsinogen. As expected from the biology of mannose 6-phosphate (Man-6-P)-containing enzyme, cathepsin D-HEL was delivered to the endosomal/lysosomal system. In contrast, the unglycosylated cathepsin D-HEL was retained in ER/Golgi and some was found in lysosomes. Most of pepsinogen-HEL was rapidly secreted from the APC. All transfectants presented HEL epitopes to T cell hybridomas. Regardless of the main route of traffic of the proteins, the strong I-Ak binding epitope HEL 48-62 was well presented by all. The biochemical forms of this epitope were identical for all. Three other epitopes of HEL that bind I-Ak with less affinity were processed equally well by unglycosylated cathepsin D-HEL and HEL-Ld. The glycosylated cathepsin D-HEL was less efficient in generating the 114-129 epitope. Pepsinogen-HEL was the less efficient of all transfectants in presenting these subdominant epitopes. Soluble cathepsin D-HEL recovered from culture supernatant was strongly immunogenic when added to C3.F6. The uptake was inhibited by free Man-6-P, indicating that the surface Man-6-P receptor can effectively deliver proteins to the class II MHC system.