Three new t-haplotypes, tp 4, tp 12 and tp 14, were isolated from M. musculus male mice captured in Central and East Bohemia, Czechoslovakia, about 400 km from the zone of hybridization between M. musculus and M. domesticus species. Complementation tests have shown that all three new t-haplotypes belong to tw 73 group. When compared with 5 t-haplotypes from M. domesticus they displayed the same pattern of BamHI restriction fragments with H-2 class I genes, and they also shared the t-specific 5·2 kb TaqI fragment of the alpha globin pseudogene. However, they differed from M. domesticus t-haplotypes at the D17Leh443 locus since they all displayed a 10·5 kb MspI fragment, labelled by the Tu443 probe, not found in wild type-chromosomes or in M. domesticus t-haplotypes. A hypothesis is proposed that t-haplotypes in M. domesticus originated by a single successful introgression from a parental species during speciation.