The effect of L-glutamate (Glu) on human lymphocyte function was studied by measuring anti-CD(3) monoclonal antibody (mAb) or phytohaemagglutinin (PHA)-induced intracellular Ca(2+) ([Ca(2+)](i)) rise (Fura-2 method), and cell proliferation (MTT assay). Glu (0.001 - 100 microM) did not modify basal lymphocyte [Ca(2+)](i), but significantly potentiated the effects of anti-CD(3) mAb or PHA. Maximal [Ca(2+)](i) rises over resting cells were: 165+/-8 and 247+/-10 nM at 3.0x10(-2) mg ml(-1) anti-CD(3) mAb; 201+/-4 and 266+/-9 nM at 5.0x10(-2) mg ml(-1) PHA, in the absence or presence of 1 microM Glu, respectively. The Glu effect showed a bell-shape concentration-dependent relationship, with a maximum (+90+/-3% for anti-CD(3) mAb and +57+/-2% for PHA over Glu-untreated cells) at 1 microM. Non-NMDA receptor agonists (1 microM) showed a greater efficacy (+76+/-2% for (S)-AMPA; +78+/-4% for KA), if compared to NMDA (+46+/-2%), or Glu itself. Ionotropic Glu receptor antagonists completely inhibited the effects of the corresponding specific receptor agonists (1 microM). The IC(50) values calculated were: 0.9 microM for D-AP5; 0.6 microM for (+)-MK801; 0.3 microM for NBQX. Both NBQX and KYNA were able to abolish Glu effect. The IC(50s) calculated were: 3.4 microM for NBQX; 0.4 microM for KYNA. Glu (0.1 - 1 mM) did not change the resting cell proliferation, whereas Glu (1 mM) significant inhibited (-27+/-4%) PHA (1.0x10(-2) mg ml(-1))-induced lymphocyte proliferation at 72 h. In conclusion, human lymphocytes express ionotropic Glu receptors functionally operating as modulators of cell activation.