The effect of PSK, a protein-bound polysaccharide and an immunomodulator, on lymphocytes was examined in vitro for 36 patients with gastric cancer and 26 with colorectal cancer. Cultured lymphocytes with PSK at 100 micrograms/ml increased the level of DNA synthesis, as determined by the 3H-thymidine uptake, from 0.9 to 3.0 fold, compared to the PSK non-treated cells. The increase was 1.36 +/- 0.46 fold for the gastric cancer cases and 1.37 + 0.45 fold for the colorectal cancer cases, and these levels were significantly the finding of 1.93 + 0.55 fold for a control group consisting of 15 healthy volunteers (P < 0.01). When the 1.3 fold increase of 3H-thymidine uptake was defined as the PSK-reactive group, 52.8% (19/36) of the patients with gastric cancer and 50.0% (13/26) for colorectal cancer were found in the PSK-reactive group. The PSK-reactive group demonstrated no relation to the age and sex of the patients, tissue differentiation type or tumor advancement. Our findings thus show that the in vitro activation of lymphocytes by PSK can help identify the candidates with either gastric or colorectal cancer who are the best suited to undergo immunochemotherapy including treatment with PSK.