The proto-oncogene HER2/neu encodes for a 185 kDa transmembrane protein with extensive homology to the epidermal growth factor (EGF) receptor. We have previously shown a correlation between HER2/neu expression and the level of in vitro cytotoxicity of tumour-associated lymphocytes (TAL) versus autologous tumour. In addition, we have recently demonstrated that tumour-associated cytotoxic T-lymphocytes (CTL) from ovarian and breast cancer patients can recognize a HER2/neu derived peptide epitope when presented in the context of HLA-A2. Since repeated tumour stimulation of CTL enhances both proliferation and cytotoxicity against autologous tumour, we hypothesized that repeated peptide antigen stimulation would have a similar effect. To be therapeutically useful, the peptide antigen must meet the following conditions: (1) the peptide must be immunogenic and cause a proliferation of CTL to adequate therapeutic numbers, and (2) the peptide-specific CTL which are generated must be cytotoxic against autologous tumour. To test our hypothesis, T-lymphocytes isolated from the ascites of four consecutive HER2/neu+ ovarian cancer patients were initially stimulated with solid phase anti-CD3 antibody and divided into three groups: (1) treatment with recombinant interleukin-2 (IL-2) alone, (2) IL-2 plus weekly stimulation with irradiated autologous tumour cells, and (3) IL-2 plus weekly stimulation with a HER2/neu derived peptide. Peptide-stimulated and tumour-stimulated CTL showed similar increases in proliferation with both groups consistently reaching therapeutic numbers. Peptide-stimulated CTL demonstrated significantly enhanced cytotoxicity against autologous tumour in 4-h chromium release assays as compared to the IL-2 alone group.(ABSTRACT TRUNCATED AT 250 WORDS)