Amino acid concentrations were analyzed in the sera of HIV (LAV/HTLV-III) positive persons and in the plasma of colorectal carcinoma patients. Both groups of persons showed significantly elevated glutamate concentrations when compared with healthy control persons. Glutamate concentrations were found to be strongly elevated in all groups of HIV-positive persons including patients with acquired immunodeficiency syndrome (AIDS) or lymphadenopathy as well as HIV-positive persons without overt symptoms, indicating that increased plasma glutamate levels may be among the earliest consequences of the HIV infection. Moreover, the increased plasma glutamate concentrations in the colorectal carcinoma patients were correlated with a decreased immunological reactivity (mitogenic responses against concanavalin A). This suggests the possibility that the increased plasma glutamate concentrations may be causally responsible for the decreased immunological reactivity in colorectal carcinoma patients as well as in patients with AIDS.