SummaryAlien H‐2k‐like antigens were found to be expressed by a methylcholanthrene induced tumour of BALB/c (H‐2d) origin. H‐2 specificities of thekhaplotype were detected on this tumour by a variety of serological techiques, including51Cr‐release cytotoxicity, microradioassay and absorption. The antisera employed were conventional polyspecific alloantisera, typing sera with restricted specificty and monoclonal hybridoma‐derived anti‐H‐2kantibodies.The tumour has a low expression of the private specificty 31, which characterizes Kdmolecules, and does not seem to express the private specificities of Dd, Kkand Dkmolecules. It appears to express predominantly alien H‐2‐like antigens which are very similar to but not identical with n