Possible antigen interactions occurring between the tumor-associated surface antigen (TASA) and the K and D regions of the major histocompatibility complex (H-2Kk,Dk) on the surface of malignant line Ib lymphocytes are currently being evaluated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and immunoelectron microscopy (IEM). Indirect immunoprecipitation of solubilized, radiolabeled membrane antigens of Ib lymphocytes employing both rat anti-H-2Kk,Dk and rabbit anti-Ib TASA and their respective antisera has demonstrated that both proteins share a 16,000 dalton moiety; thus, suggesting the "altered-self" model at the target cell level. These observations are currently being confirmed by direct observation at the ultrastructural level employing a double-labelling technique. Studies concerning the distribution and valence of the Ib TASA indicate that the antigen is randomly distributed on all malignant lymphocytes and occupies about 1% of the total surface area. Similar analysis of the distribution and valence of the H-2Kk,Dk antigens by IEM have been initiated but are too preliminary for comment.