A new method for typing minor H antigens is described. The method is based on the observation that effector T cells, sensitized in vivo to minor H antigens and then rechallenged in vitro, will lyse labeled target cells in the CML assay, provided that the target, the sensitizing, and the responding cells share the same allele at H-2K or H-2D loci. In this method, two congenic lines differing at a minor H locus are cross-immunized (by skin grafting, injection of lymphoid cells, or, preferably, by a combination of both treatments), rechallenged in vitro, and tested against a panel of strains carrying the same H-2 haplotype as the two lines. Target cells of the panel carrying the same minor H antigens as the strain against which the effector cells were sensitized are lysed; target cells not carrying these antigens are unaffected. The feasibility of the method was demonstrated by testing for H-3 antigens of a panel of H-2b-bearing strains. The new method is faster and more economical than the classical F1 method which has been used until recently.