The immune response may well be a limiting factor to the therapeutic efficacy of the vaccine. If this is the case, it then becomes crucial to understand why some patients develop a cytolytic T lymphocyte (CTL) response against the vaccine, while the majority of them does not so. One possible explanation for the low frequency of clinical responses is that each injection of a single peptide has a low probability to provide the adequate stimulus to activate very rare CTL precursors. This probability should be increased if several peptides known to be undoubtedly associated with tumor regressions were used together to immunize patients.