Chronic Myeloid Leukaemia (CML) is a malignant clonal disorder of haematopoietic stem cells characterised by a reciprocal chromosomal translocation t(9; 22) that leads to the formation of a novel hybrid BCR-ABL fusion gene. One of the two major types, b2a2 or b3a2, is found in most CML patients. Peptides derived from the fusion junction of BCR-ABL can potentially act as a target for T lymphocyte-mediated immune responses. CD4⁺ helper T-lymphocytes have been shown to play a vital role in anti-tumour immune responses but only a few epitopes of tumour-associated antigens have been recognised by HLA class II restricted CD4⁺ T-lymphocytes. Little information is available about CD4⁺ T-helper responses in CML patients. The aim of this study was to investigate T-cell responses against the b3a2 23-mer VHSATGFKQSSKALQRPVASDFE, the 9-mer HLA-A3 restricted KQSSKALQR, and HLA-B8 restricted GFKQSSKAL BCR-ABL peptides. Peripheral blood mononuclear cells (PBMCs) and dendritic cells (DCs) were used as antigen presenting cells (APCs) for CD4⁺ T-cell responses. T-cell blasts (TCBs) were used as APCs for CD8⁺ T-cell responses. CD4⁺ responses were assessed by tritiated thymidine incorporation assay and intracellular cytokine staining. CD8⁺ responses were assessed by chromium release assay and tetramer analysis. The results of this study revealed that no specific proliferative responses were obtained in most normal subjects and CML patients. In both groups, repeated in vitro stimulation with the 23 mer BCR-ABL peptide on two, three or four occasions using pulsed PBMCs or dendritic cells revealed no significant responses to the 23 mer BCRABL peptide despite significant responses being obtained against recall antigens in both groups. No specific CD4⁺ T-cells producing IFN-γ were detected in either normal subjects or CML patients in response to repeated in vitro stimulation with the 23-mer BCR-ABL peptide, despite a significant response to influenza peptide in CML patients. No specific CD8⁺ T-cells were detected in HLA-A3⁺ and HLA-B8⁺ normal subjects or CML patients as assessed by chromium release assay and tetramer analysis. While chromium release showed cytotoxic T-lymphocyte (CTL) expansion in one of seven normal subjects and in one of five CML patients, tetramer assessment did not confirm a specific expansion in either group. The discrepancy between the results of these two techniques might be due to lower sensitivity of the tetramer assay, or because tetramers may detect non-functional CD8⁺ T-cells. The results suggest that there may be a defect in the CD4⁺ responses to this particular BCR-ABL peptide. Previous work using shorter peptides has shown that CD4⁺ T-helper responses can be elicited in normal subjects. However, in CML patients there may be an absence of circulating BCR-ABL specific CD4⁺ T-helper precursors. As b3a2 specific class I HLA restricted CD8⁺ T-cell responses are detectable in some normal subjects and CML patients, there may be a defect in b3a2 specific CD4⁺ helper T-cell response that prevents effective anti-leukaemic responses in vivo. Stimulation of BCR-ABL fusion peptide-specific CD4⁺ T-cells may be achieved by in vivo peptide vaccination that may offer promise for novel immunotherapy protocols in CML patients.