SUMMARYTwo new forms of Mta, the maternally transmitted antigen of the mouse, were defined by cytotoxic T lymphocytes. One is found in laboratory mice carrying the β allele of the maternally transmitted factor,Mtf, which were previously thought to be Mta‐negative. The other is associated with a new allele,Mtfγ, and is found in wild mice from Toulouse. Like the common form of Mta, determined by the extrachromosomalMrfα, the new forms are also dependent on the α allele ofHmt, a chromosomal gene linked toH‐2. and are associated with β2‐microglobulin. Killers could be raised in all six combinations of a, β, and γ mice, showing that each form ofMtfdetermines a distinct (set of) epitope(s). We propose that the product of theHmtgene, a class I MHC antigen, presents a peptide derived from a mitochondrial gene product on the cell surface and thereby creates the target