The genes encoding the Lewis rat RT1.B molecule (MHC Class II I-A equivalent) were transfected and expressed in mouse DAP.3 fibroblast cells together with the gene encoding the mouse ICAM-1 molecule. Both molecules were stably expressed on the cell surface of DAP.3 cells under longterm culture conditions. The RT1.B/mICAM-1 transfectants presented antigen in a specific manner to a RT1. B-restricted rat T cell hybridoma specific for the 69-89 peptide of myelin basic protein (BP). In addition, the transfectants were able to present antigen to a BP69-89-specific rat T cell line. Presentation to a RT1.D (MHC Class II I-E equivalent)-restricted BP87-99-specific T cell line was minimal. Production of the Th1 cytokine IFN-gamma by BP69-89-specific T cells when stimulated by RT1.B/mICAM-1 transfectants correlated very well with proliferation to specific antigen. Moreover, RT1.B-transfected DAP.3 cells sufficiently stimulated BP69-89-specific T cells such that they were able to transfer experimental autoimmune encephalomyelitis (EAE) to Lewis rat recipients. Thus, the RT1.B molecule is functionally expressed on the surface of transfected Dap.3 fibroblasts and is capable of MHC Class II-restricted, antigen-specific presentation to rat T cells.