Enteral or parenteral arginine supplementation enhances lymphocyte activation after mitogenic stimulation, in rats and humans. Arginine deprivation in culture media is associated with a reduction of lymphocyte activation; this effect, however, has not yet been proven to be specific for arginine. This study was designed to evaluate the specificity of arginine deprivation from culture media on the reduction of in vitro lymphocyte activation. Peripheral blood mononuclear cells, obtained from 11 healthy volunteers, were cultured in RPMI 1640, selectively deprived of single amino acids (i.e. arginine, phenylalanine, leucine, methionine, and threonine) and supplemented with phytohemagglutinin (PHA). The expression of interleukin-2 receptor and transferrin receptor was evaluated by cytometric analysis; the levels of soluble IL-2 receptor were determined by immuno-enzymatic assay. Results were compared with those obtained by culturing cells in non-deprived, RPMI 1640 medium. The expression of transferrin and IL-2 receptor, as well as the levels of IL-2 soluble receptor, were significantly reduced in all deprived media irrespective of the lacking amino acid. These results suggest that the reduction of in vitro lymphocyte activation is not an arginine specific effect. Therefore, the known enhancement of immune response, following arginine supplementation in vivo, is likely to involve a more complex series of events.