We isolated dendritic cells (DC) from lymphoid organs of mice bearing a transgene for a membrane-bound form of the model protein hen egg white lysozyme (HEL). DC from the spleen had a lower representation of costimulatory molecules and class II MHC molecules than those isolated from lymph nodes and thymi. Splenic DC were capable of further maturation by in vivo treatment of mice with LPS. The immature DC from spleen processed HEL and displayed the chemically dominant epitope as evidenced by FACS analysis. These immature DC also presented this epitope to CD4+ T cells. Splenic DC from another transgenic mouse (ML-5) containing serum HEL also showed the ability to process and present Ag despite low levels of circulating HEL. In vitro-derived DC from the bone marrow (bone marrow-derived DC) of mHEL mice also displayed immature to mature features and in both cases displayed HEL peptides as well as SDS-stable MHC class II molecules. Immature bone marrow-derived DC also processed exogenous HEL. We conclude that the DC sets normally found in tissue show a scale of maturation features but even the most immature process and present peptides; by MHC class II molecules.