Soluble interleukin-2 receptor (sIL-2R) levels have been found to be elevated in several clinical conditions, including disseminated solid neoplasms, whereas they are generally within the normal range in patients with locally limited neoplastic disease. The aim of the present study was to examine this in our colon cancer patients, and to assess if this situation can affect the in vitro activation of peripheral blood mononuclear cells (PBMC), examining the proliferative response to IL-2 and anti-CD3 monoclonal antibody, the IL-2 serum levels and the PBMC phenotype. The results show that sIL-2R levels were significantly correlated with the stage of the disease, showing an increase from stage I to stage IV; moreover, it is worth noting that the proliferative response to IL-2 plus anti-CD3 is significantly higher than to IL-2 alone in stage IV, without significant alteration in the numerical presence of T and natural killer cells. So it seems that in the peripheral blood of patients, connected with the disease progression, are present cellular populations showing a different response to activation, and that T cells acquire a better response condition than NK. Thus, since the T cellular population includes the tumour-specific cytotoxic precursor cells, this should be helpful for its tumour regressive activity, but it is conceivable that this population cannot perform its functions, owing to a deficiency in responsiveness of the specific ThCD4+ subpopulation.