The embryo-derived teratocarcinoma TC1Ph of the genotype (129 t0/T X C3H)t0/+ and embryonal carcinoma (EC) cell line EC1Ph, established from the TC1Ph, were used as grafts for mice differing only at the T-H-2 interval of chromosome 17. Using exceptional recombinants in this chromosome region a two-gene model of genetic control of EC histocompatibility was proposed. The Cech-1 gene situated at the T end of the T-H-2 interval would control expression of the EC histocompatibility gene located near the H-2 complex.