Antifolates have demonstrated effective antineoplastic activity in the treatment of disorders of cell proliferation, e.g., acute lymphocytic leukemia, breast cancer, and mycosis fungoides. The enzymatic pathways involved in DNA biosynthesis, specifically dihydrofolate reductase and thymidylate synthetase, are the biochemical targets of antifolates. Methotrexate (MTX) and its analogs, 10-ethyl-10-deazaaminopterin (edatrexate), and trimetrexate (TMT) are paradigms for cytotoxicity at the biochemical level. Understanding the cellular pharmacology of MTX and other antifolates has provided a strong rationale for the use of high-dose MTX with leucovorin (LV) rescue. The combination of MTX and LV prevents severe toxicity without diminishing the antitumor activity of the drugs. The efficacy of antifolate drugs is related to the extent of intracellular polyglutamation in normal and cancer cells. Since toxicity in patients is difficult to predict, monitoring drug concentrations is critical. Antifolates, specifically MTX and edatrexate, are among a growing class of chemotherapeutic agents that require assiduous and rapid monitoring to help prevent severe systemic toxicity. Chemical and physical properties, mechanism of chemotherapeutic activity, and analytical methodology for measurement of serum concentrations of antifolates will be discussed.