SUMMARYThe murine Ly‐4 locus is further described with the definition of both Ly‐4.1 and 4.2 specificities and the demonstration of linkage ofH‐3 on Chromosome 2. As has been found for several other Ly‐specificities, both Ly‐4 specificities are absent from the thymus, but are found on peripheral T and B cells, with B cells having the greatest antigen expression. The expression of both Ly‐4 specificities was influenced by an H‐2‐linked gene, mapping in or near