We tested the hypothesis that the loss of immunological reactivity in HIV-1-infected persons may result partly from a virus-induced metabolic disorder. Patients who are infected with the acquired immunodeficiency syndrome (AIDS)-associated human immunodeficiency virus 1 were found to have, on average, markedly elevated and highly variable plasma glutamate concentrations. A similar elevation of the extracellular glutamate concentration was found to inhibit DNA synthesis in cultures of mitogenically stimulated lymphocytes. An even stronger inhibition was seen with the structural analogue quisqualate, and a moderate inhibition was seen with N-methyl-D-aspartate and kainate, i.e. with well established pharmacological probes for the excitatory glutamate receptors in the vertebrate central nervous system. The inhibitory effect of glutamate was compensated by adding cysteine or relatively large numbers of 'splenic adherent cells' to the culture. Elevated extracellular glutamate levels were also found to reduce the capacity of murine macrophages, human blood monocytes, and murine fibroblastoid cells (L929 cells) to release acid-soluble thiol (cysteine) into the extracellular space. The three cell types differed, however, with respect to their sensitivity against the three structural analogues of glutamate. The elevated glutamate concentration was not non-specifically toxic for cultured macrophages, since glycolytic activity and arginase activity were not inhibited. Implications of these observations for the pathogenetic mechanism of AIDS are discussed.