Tumor cell lines have been identified which express Ia antigens. These tumor cells will generate antigen-specific T-cell proliferation when assayed with continuous cultures of soluble antigen-reactive T cells in an MHC restricted fashion. We will utilize this model system of antigen presentation to characterize biochemically how a defined soluble protein antigen, myoglobin, is processed by these antigen-presenting cells. With the use of bifunctional reagents, we will examine the physical association between Ia antigens and myoglobin or myoglobin fragments on the antigen presenting cell membranes. We will also use this model system to try to produce and characterize biochemically soluble Ia antigen-myoglobin complexes. Finally, we will utilize the cell surface markers common to these tumor cells to isolate normal lymphocyte populations which have the same functional properties as these tumor cells. The characterization of this model system will be used to identify the biochemical events that occur during the initial stages of an immune response to soluble foreign antigens.