SummaryCellular cytotoxicity experiments were done to test the potential of the extra H‐2 antigenic specificities on the K36 spontaneous leukaemia as targets. In addition, experiments were performed to rule out the possibility that the target determinants could be normal cross‐reacting alloantigens, e.g., T1, Qa, non‐H‐2 and previously undetected H‐2 public specificities, which are more readily detectable on tumours than on normal cells.We used F1hybrid mice, in which one parent was of the strain of origin of the tumour (K36) and the other parent of the B10 congenic series, i.e., (AKR x B10)F1. These cells were stimulated by lymphoid cells from other B10 congenic strains, B10.A and B10.D2, and tested against the test tumour K36 and several PHA blast controls. Several K36 sublines as well as a cloned line of K36 (K36.16) were used and significant cytotoxicity against. an H‐2d‐like target on these tumour cells was obtained. These data exclude the possibility of a corss‐reactive alloantigen, e.g., undetected H‐2 public specificity, or differentiation antigens. These results with the K36 tunour were consistent with our immunochemical studies (see Schmidt&Festenstein, 1980) and were confirmed and extended by cold target inhibition experiments. In these experiments, B10.BR cells were sensitized by B10.D2 lymphoid cells and tested against B10.D2 (51chromium‐labelled PHA balsts). Two kinds of normal unlabelled lymph node suspensions as well as the K36.16 tumour cell suspension were used. Significant specific inhibition of between 19% and 40% was obtained using K36 and between 23% and 37% using B10.D2 (positive conrol). AKR cells (negative control) in contrast were unable to reduce the precentage specific cytotoxicity.Since it was already known that theH‐2Kkgene products are missing from this tumour (Schmidt&Festenstein, 1980), it was of interest to test whether cytotoxic effectors directed against theH‐2kkgene products were able to kill the K36 tumour. Accordingly, B10.D2 lymphoid cells were sensitized to B10.BR (C3H.0H) and B10.A targets, respectively, and tested against K36 and appropriate controls. Only weak killing was observed when sensitization was effected against the K end of theH‐2khaplotype (i.e., using B10.A as the sensitizing cell) but strong and significant cytolysis was found when the sensitization was against the wholeH‐2khaplotype or against the H‐2Dkgene product. These results were confirmed by cold target inhibition studies.These experiments provide further indications for the H‐2d‐like characteristics of these allodeterminants. We have already excluded some of the possible explanations for these findings (i.e., corss‐reactions with H‐2 and non‐H‐2 normal specificities). The cold target inhibition experiments rule out non‐specific viral effects. Thus, we favour an alteration in regulatory genes leading to repression of theH‐2Kkproduct and derepression of theH‐2Ddproduct, but cannot formally rule out highly corss