The mechanisms underlying the impaired immune response frequently observed in cancer patients are not fully understood. Alteration of T-cell-associated signal transduction molecules has recently been implicated in immune suppression in tumor-bearing hosts. Furthermore, T cells from tumor-bearing host, irrespective of the presence of the zeta-chain, showed a lack of proliferative activity and cytotoxic function. In the present study, we investigated the expression of the zeta-chain molecule and the p56(lck) and p59(fyn) protein tyrosine kinase (PTK) levels in peripheral blood T lymphocytes (T-PBL) from patients with advanced gastric carcinomas; for this, flow cytometric analysis and immunoblotting, respectively, were used. We also compared the results of flow cytometric analysis of PBL between stomach cancer patients and normal healthy volunteers. In T-PBL from 22 tumor-bearing hosts, significantly reduced zeta-chain expression (16/22, 73%) was observed. Moreover, the expression level of p56(lck) in T-PBL from patients was significantly lower than that of p59(fyn). Flow cytometric analysis of T-PBL indicated a markedly increased CD8+28- cell population in T-PBL from 19 tumor-bearing hosts.