The H-2K glycoproteins from two mouse mutants B6.C-H-2ba (Hzl) and B6-H-2bd (M505) were isolated, and by tryptic peptide mapping techniques, were compared with each other and with the H-2K glycoprotein from the parent strain C57BL/6 (H-2b). The elution profile of the acid-soluble tryptic peptides from the H-2Kba glycoprotein showed that at least two to three arginine-labeled peptides and one lysine-labeled peptide were different from the H-2Kb product of the parent. These peptides were distinct from the soluble peptides which constitute the difference which was found between the H-2K products of H-2bd and the H-2b parent strains. The comparative peptide maps thus demonstrate small but significant primary structural differences among the H-2Kba, H-2Kbd, and H-2Kb glycoproteins. The findings are in agreement with the suggestion that alterations in the H-2K products from these two mutants are sufficient to permit their cells to stimulate each other and the parent cells in the mixed lymphocyte reaction (MLR) and to serve as target cells for the cell-mediated lymphocytotoxicity (CML) test. Because there is a significant reactivity in MLR and CML between H-2Kb, H-2Kba and H-2Kbd cells whereas there is no serologic reactivity, our findings are consistent with the notion there is a separation of the site(s) on the H-2 molecule having serologic reactivity and the site(s) involved in the MLR and CML reactivities.