SUMMARYAn alloantiserum prepared against LyM‐1 antigen of the mouse has been analysed in detail. This antiserum, C3H/HeJ anti‐CBA/J, has turned out to define two antigens which are specified by independently segregating loci. One of these is Lyb‐2.1 or similar antigen specified by a gene located on chromosome 4. The other is identified as LyM‐1.2 based on the result that a gene determining alloantigenicity in the Lyb‐2‐strain is closely linked toMlswith recombination frequency of 0.066 ± 0.028. Since both antigens are expressed preferentially on B lymphocytes and their strain distribution is significantly overlapped, there has been much confusion about LyM‐1 antigen. This is overcome by re‐evaluating anti‐LyM‐1.2 serum under the condition in which Lyb‐2.1 is not involved in analysis. The strain distribution of LyM‐l antigen, its preferential expression on B lymphocytes and close genetic linkage ofLyM‐l