The maternally transmitted antigen (Mta) is expressed on the cells of most strains of mice. It is a medial histocompatibility antigen, that is, it is recognized by unrestricted cytotoxic T lymphocytes as are major H antigens, but unlike these it is a weak transplantation antigen and does not itself restrict the T-cell recognition of minor H antigens. All other medial H antigens are encoded by genes closely linked to the major histocompatibility complex, H-2 in the mouse. By contrast, Mta appeared to follow extrachromosomal, maternal inheritance. Several substrains of NZB, NZO and non-inbred European NMRI mice are Mta-negative. Females of these strains bear only Mta- offspring, while females of the inbred Mta+ strains bear only Mta+ offspring. Repeated backcrossing from Mta+ females to NZB or NMRI males has shown that, given the right cytoplasmic genes, the chromosomal genes of these Mta- strains permit expression of Mta2. As the Mta type of a mouse cannot be influenced by embryo transfer or foster nursing, we concluded that it was determined by a cytoplasmic factor (Mtf), transmitted through the egg. We now show that a gene, Hmt, closely linked to the H-2 complex, is also required for expression of Mta.