The 2.0 kb-long nucleotide sequences of the promoter regions of two closely related class I genes of the mouse major histocompatibility complex (H-2Kb and H-2Kbm1) have been determined and compared. The promoter sequence of the H-2Kbm1 gene differs from that of the H-2Kb gene by a single deletion of a 'C' at position -456 in the upstream region of H-2Kbm1 gene. The actual existence of this deletion of a single base in genomic DNA has been verified by genomic DNA hybridization, using oligonucleotide probes specific for H-2Kbm1 or H-2Kb respectively. The effect on the enhancer activity of H-2Kbm1 promoter region of the difference at position -456 has been analyzed by the chloramphenicol acetyltransferase (CAT) assay, using appropriate DdeI fragments (-533 to -408 for H-2Kbm1; -534 to -408 for H-2Kb) cloned downstream of pH-2(367)CAT gene construct. The CAT activity determined by the H-2Kbm1 fragment was about 3-fold higher than that of H-2Kb, a result which probably accounts for the higher level of the H-2Kbm1 transcript and antigen in lymph node cells.