sanchez and personnaz were treated with anti class ii monoclonal antibody 949 or the supernatants for cos cells transfected with dna coding either for lag 3 d1 d4 ig or for cd8 ig the five cell lines expressing the different haplotypes of mhc class ii molecules were recognized by lag 3 ig in the same way as by the anti class ii monoclonal antibodies positive control while the supernatant containing cd8 ig negative control did not bind to these cell lines as could be expected four mhc class ii negative cell lines cem rj hsb2 k562 were treated with the same reagents as above none reacted either with the anti mhc class ii negative control or with lag 3 d1 d4 ig showing that the binding of lag 3 d1 d4 is specific to mhc class ii molecules further experiments were carried out using i mouse fibroblasts transfected or otherwise with genes coding for human dr7 or human dp4 ii mouse cells expressing or otherwise mhc class ii molecules iii activated human cd4 or cd8 cells and iv t lymphocyte lines expressing the different haplotypes of mhc class ii molecules fig8 unlike cd8 ig lag 3 d1 d4 ig binds to all cells expressing mhc class ii as efficiently as the anti mhc class ii monoclonal antibody 949 lag 3 d1 d4 ig binds to all dr and dp haplotypes tested to human mhc class ii molecules expressed by transfected mouse cells to murine mhc class ii molecules and also to mhc class ii molecules expressed by cd4 or cd8 t lymphocytes.