During an LCM virus infection in mice, cytotoxic T lymphocytes appear to be directed against target cells infected with the virus. This cytotoxic reaction is restricted by the H-2 histo-compatibility between effector and target cells. To analyse this restriction, the cytotoxic effect of in vivo sensitized spleen cells from mice of various H-2 specificities on LCM virus-infecte of in vivo sensitized spleen cells from mice of various H-2 specificities on LCM virus-infected L cells (H-2k) was studied in vitro. When spleen cells from strains identical with C3H (H-2k) and L cells at the K as well as at the D locus were used as effector cells, the degree of lysis was of the same order as that obtained by C3H effector cells, whereas strains different at both loci did not respond significantly. The A/J and B10A strains, both identical with the L cells at the K locus but different at the D locus, gave low or moderate responses. C3H-H-20 lymphocytes, identical with the targets at the D but not at the K locus, showed high degrees of cytotoxicity although the level obtained by strains histocompatible with L cells at both loci was never reached. Effector cells from the F1 hybrids C3H/HeJ X BALB/c and C3H/HeJ X DBA both responded poorly in spite of the fact that one of their haplotypes carries the same marker loci as the L cells. This finding is in contrast with that obtained in other laboratories where the LCM virus system and other systems were studied. A point is made of the design and the evaluation of the experiments.