Initial rates of [³H]MTX uptake in CCRF-CEM cells and their various PT523-resistant sublines. Exponentially growing cells were washed, resuspended in a HEPES-buffered saline solution at pH 7.4 containing 2 μM [³H]MTX, and incubated at 37°C for 3 minutes. Cells were then washed 3 times with ice-cold buffer, centrifuged, lysed, and processed for scintillation counting as described in “Materials and methods.” Influx rates are given as pmol [³H]MTX/min/107 cells and are presented as the means ± SD of 3 to 6 independent experiments performed on different days. The influx rate of [³H]MTX in parental (WT) cells was 3.9 ± 0.64 pmol [³H]MTX/min/107 cells. The influx rate of WT cells was referred to as 100% and the residual percentage of [³H]MTX transport in the various sublines relative to parental cells is also depicted.