The B10.SAA48 congenic line was derived by transferring theH‐2 haplotype of a wild mouse onto the background of the inbred strain C57BL/10Sn (abbreviated as B10). The line carries theDw3allele in combination with an L allele different from that present in otherDw3strains. One possible explanation of this finding is that crossing over occurred between theDandLloci. The determinants H‐2.m64 and H‐2.m65 represent an inclusion doublet in which the latter never occurs without the former. Typing of B10.W lines with Ld‐specific CTLs reveals the absence of this allele in all the lines, including those carrying an allele serologically simil