Structural abnormality of T cell receptor (TCR)-CD3 complex on the cell surface was investigated in peripheral blood lymphocytes (PBL) from 55 cancer patients. In 24 of the 68 tests done on these patients, the CD3 zeta chain was not detected by immunoprecipitation with anti-CD3 epsilon monoclonal antibody (mAb), but was observed with anti-CD3 zeta mAb, suggesting that a high frequency of cancer patients possesses abnormal T cell receptor (TCR) complex in PBL. On the other hand, the total zeta chain was missing in several advanced cases. During follow-up of several patients, the zeta chain became undetectable after two or three months of cancer progression. It appears that immunosuppressive status can be monitored by analyzing the TCR-CD3 complex on the cell surface of PBL.