Loss of T cell-associated signal transduction molecules has recently been implicated in immune suppression in tumor-bearing hosts. In the present study, we have examined this and related phenomenon extensively in a large number of tumor-bearing mice, analyzed individually. Splenic T cells from tumor-bearing mice were isolated and characterized with respect to the following: (a) levels of three tyrosine kinases, p56lck, p59fyn, and ZAP-70; (b) expression of CD3-zeta; (c) alloreactive responses; and (d) antigen-specific responses. Contrary to recent reports, T cells from tumor-bearing mice were observed to express normal levels of lck, fyn, ZAP-70, and CD3-zeta. Further, T cells showed healthy alloreactive and antigen-specific responses until approximately 3 wk after post tumor challenge, when the tumors constituted approximately 20% of the body weight. Alterations with respect to some parameters were observed only in mice that had been bearing larger tumors for a considerably longer period. As human tumors are unlikely to grow to such large sizes (e.g., > 20% of the total body weight), the significance of the alterations in T cell expression of lck, fyn, ZAP-70, or CD3-zeta in the immune status of cancer patients is unclear. Altogether, these results indicate that alterations in T cell signal transduction molecules do not account for the profound tumor-specific suppression observed during tumor growth.