- Reciprocal transplantation of D-region-histoincompatible thyroid grafts between mice of the B10.S and B10.S(7R) strains resulted in acceptance of all B10.S thyroid grafts by B10.S(7R) recipient mice and acute rejection of 93% of the thyroid grafts in the opposite direction. Evidence of expression of D region gene products on the surface of cells of B10.S thyroid grafts was supported by acute second-set rejection of B10.S thyroid grafts by B10.S(7R) recipients preimmunized with a B10.S skin graft. In vitro analysis of reciprocal immune responses between B10.S and B10.S(7R) mice demonstrated that the unidirectional thyroid graft rejection correlates with a relatively high frequency of cytotoxic T lymphocyte precursors (CTL-P). The frequency of B10.S splenic CTL-P reactive with B10.S(7R) (1/1131) was similar to that against the entire H-2 difference (i.e., B10.S anti-B10.BR, 1/776). However, B10.S anti-B10.S(7R) CTL-P frequency was about 5-fold higher than the reciprocal CTL-P frequency of B10.S(7R) anti-B10.S. The correlation between the high CTL-P frequency and the ability to reject thyroid grafts, as well as possible causes for the difference in the reciprocal CTL-P frequencies are discussed.