Elevated glutamate concentrations are commonly observed in tumor patients, and glutamate was recently found to competitively inhibit the membrane transport of cystine. We therefore investigated the possibility that elevated plasma glutamate levels may damage the immune system. The experiments in this report demonstrate a link between the individual plasma glutamate level and the individual immunological reactivity as measured by mitogenic responses. This correlation has been analyzed in 39 colorectal carcinoma patients, 40 oat cell carcinoma patients, 24 large cell lung carcinoma patients, and 31 apparently healthy persons (blood donors). Blood cells from all three groups of tumor patients in comparison with cells from healthy persons produced markedly reduced mitogenic responses against PWM, and all three groups of tumor patients had on average significantly elevated plasma glutamic acid concentrations. Our analysis revealed a linear regression of the logarithm of the individual plasma glutamate levels (before therapeutic treatment) on the logarithm of the corresponding mitogenic reactivity against PWM for the entire population of 134 persons tested (correlation coefficient -0.80; level of significance P less than 0.00001). A statistically significant linear correlation with a similar regression equation was also observed in the group of the healthy blood donors (n = 31; correlation coefficient -0.56; P less than 0.01), indicating that this correlation is universal and not dependent on the presence of a tumor. Mitogenically stimulated murine lymphocyte cultures revealed an inverse correlation between glutamate concentration and cell proliferation in response to the mitogens PHA and PWM.