The response of A-strain females to the male-specific antigen is very weak; both first- and second-set test skin grafts are rejected protractedly and about 50% of the grafts are retained in spite of generally present morphologic signs of cellylar immunity induced by these grafts. When the first-set skin graft involved H-incompatibilities additional to the MSA (such as H-2 only or H-2 plus multiple non-H-2) all the second-set grafts, which were only MSA-incompatible, underwent a markedly accelerated rejection. The possible mechanism of this adjuvant-like effect is discussed.