In the cell-mediated lymphocytotoxicity assay, A.TH effector cells sensitized to A.TL lymphocytes lyse not only A.TL but also C3H/HeJ and B10.A(4R) targets. Similarly, B10.T(6R) anti-B10.AQR effector cells lyse B10.BR and B10.BYR target cells in addition to B10.AQR cells. These findings indicate that for CML to occur across the I A region barrier, compatibility at K or D regions is not required. © American Society for Clinical Pathology.