The Thymus leukemia antigen (Tla) locus is defined serologically by antigens confined to immature cortical thymocytes. It lies to the right of H-2D at a distance of 1.5 centimorgans. The original discovery of Tla in 1962 was followed by recognition that it is complex, i.e. determines several specificities, and then by the finding that there are Histocompatibility loci in the Tla region, defined as the segment of chromosome 17 from a point immediately to the right of H-2D to some point beyond Tla. Highly sensitive cytotoxicity assays now reveal that genes in the Tla region code for surface components on peripheral T cells. These are decisively distinguished from the known TL antigens of thymocytes by the two new cross-over congenic stocks B6.K.1 and B6.K.2 developed by Flaherty. We propose an extensive immunization program and renewed survey of antisera in our collection to delineate further the immunogenetics of the Tla region as it concerns peripheral functionally mature T cells. The object is two-fold: (1) to provide further information on the fine genetic structure of the Tla region, especially in view of its proposed homology with its neighbors H-2D, H-2K and T/t, and (2) especially to investigate and develop such new systems coded by genes in the Tla region as may serve in the discrimination among functionally distinct T cell subclasses.