Recognition of leukemic cells by leukemia-reactive CTL or an anti–HLA-A2 CD8⁺ control CTL clone. CTL were preincubated with anti-CD4 or anti-CD8, or leukemic target cells were preincubated with anti-HLA class I or class II monoclonal antibodies for 30 minutes. The percentage of lysis was then measured in a⁵¹Cr-release assay at an effector:target ratio of 3:1. Recognition of leukemic cells by the leukemia-reactive CTL was blocked by anti-CD4 or anti-class II antibodies.