HCL-cell–specific recognition by the T-cell clones was restricted by HLA class II. The restriction of recognition of the HCL cells by the T-cell clones was determined by blocking studies. The HCL cells from patient A (white bar) and B (black bar) were incubated with saturating concentrations of pan-HLA class II (PdV5.2), HLA-DR (B8.11.2), HLA-DQ (SPVL3), or HLA-DP (B7.21) MoAbs for 15 minutes before the responder T cells were added. The specific proliferative responses of the BV2⁺ T-cell clones 4F10 (A) and 1F2 (B) were blocked by an HLA-DQ MoAb. The BV8S3⁺ T-cell clone (C) recognized the HCL cells in an HLA-DP–restricted way.