The present study was designed to assess the effects of the protein-bound polysaccharide PSK on the immunological status of patients with gastrointestinal cancer. Twenty-nine gastric and 18 colorectal cancer patients were randomly assigned to either the control or PSK group. Patients in the PSK group were given 3.0 g of PSK orally before surgery, either daily or every other day. Patients in the control group received no PSK. The data of peripheral blood lymphocytes (PBL) were compared before and after administration of PSK, and those of the regional node lymphocytes (RNL) were compared between the control and the PSK group. The results indicate that the effects of PSK were significantly influenced by the duration of administration, but not by the frequency of administration. In the patients belonging to the short term PSK group (administration less than 14 days), the response of the PBL to PSK and Con A become significantly stronger compared to before the administration of PSK, whereas the cytotoxicity against K562 and KATO-3, and the proportion of CD16+ cells increased significantly in those patients belonging to the long term PSK group (greater than or equal to 14 days). In addition, the proportion of CD9 + 11b + suppressor T cells decreased in the RNL of the short term PSK group, whereas the proportion of CD4 + Leu8 - helper T cells in the RNL increased in the long term PSK group. These results suggest that the oral administration of PSK leads to the suppression of suppressor cells in the RNL.(ABSTRACT TRUNCATED AT 250 WORDS)