SUMMARYSeven mouse tumour cell lines of different aetiology and origin were tested for the expression of surface alloantigens using twenty‐four well defined H‐2 allo‐antisera and anti‐Thy 1.2. We used a new radioassay that involves antibody‐complement treatment of the tumour target cells followed by postlabelling the surviving tumour cells with14C‐thymidine. With a relatively high frequency the anti‐H‐2 sera were reacting differently with the tumour cells than with respective tive syngeneic lymphoid cells. Thirty‐six anomalous reactions out of 129 investigated were detected. Absorbtion experiments performed with H‐2 antigen positive or negative lymphoid cells revealed a striking similarity between these extra‐specificities and H‐2 specificities of foreign haplotypes. The results are discussed in relation to the biological importance of the extra‐specificities, both with regard to origin (derepression) and function (transplant