Correlation between TSIA and antifolate growth inhibition measured by the MTT assay for wild-type CCRF/CEM leukemia cells and three MTX-resistant sublines. TSIA and MTT-based growth inhibition were determined for MTX (•) and five other antifolates (AG337, edatrexate, GW1843U89, trimetrexate, and ZD1694) (⧫). The TSI₅₀ values, measured by the 21-hour incubation TSIA, and the IC₅₀ values after 3 days of culture in the MTT assay were significantly correlated (r = .89, P < .001).