Mechanisms of the discrepant effect of methotrexate and dipyridamole on human hematologic cultured cell lines were investigated by analyzing intracellular methotrexate levels and thymidine incorporation through the salvage pathway, since the combination of methotrexate and dipyridamole has different effects according to cell type: additive effects on ML-1 and THP-1 (myelo-monocytoid cells); reduced effects on MOLT-3, SKW-3, P32/ish and BL-TH (lymphoid cells). Dipyridamole reduced the toxicity of methotrexate by diminishing intracellular methotrexate levels in MOLT-3 and BL-TH (lymphoid cells), in which the reduction of intracellular methotrexate affected more than just the blocking of the salvage pathway required for growth by dipyridamole. On the other hand, dipyridamole enhanced the toxicity of the combination by blocking the salvage pathway in an ML-1 (myelo-monocytoid cell) and in a methotrexate-resistant subline of BL-TH/MTX (lymphoid cell), in which the salvage pathways were considered activated. Dipyridamole could prove to be a useful drug for reversing the drug resistance caused by the activation of the salvage pathway.