We examined the progressive and irreversible loss of antigen-specific lymphoproliferative responses in peripheral blood mononuclear cells (PBMC) obtained from blood exposed for prolonged periods to EDTA as an anticoagulant. The responses of these lymphocytes to interleukin-2 or to concanavalin A were, however, unaffected. The observed loss was not due to depletion of metal ions by EDTA, since the addition of several divalent cations to whole blood during storage in EDTA or to lymphocytes from EDTA-stored blood during antigen stimulation in vitro did not alleviate the defect. Reconstitution of antigen-specific T-cell lines or Percoll-purified T cells with adherent antigen-presenting cells in antigen stimulation assays revealed that the presenting cells and not the effector T-cells were the targets of EDTA-mediated damage. The anticoagulant heparin helped to circumvent this problem. Surprisingly, EGTA, another metal ion chelator, could successfully replace EDTA, with a marginal loss in antigen-specific responses. Lymphoproliferative responses to antigens of Japanese encephalitis virus (JEV) and Mycobacterium tuberculosis were both significantly preserved in EGTA. JEV antigen-specific responses of PBMC obtained from the blood of convalescent JEV patients and stored in EGTA for as long as 24 h (n = 20) were comparable to those of fresh PBMC (n = 10), while PBMC from blood stored in EDTA (n = 17) for 16 h or longer failed to respond. We recommend that EGTA be used as the anticoagulant of choice for applications that require the lymphocyte proliferation assay, especially when on-site testing facilities are not available.