DHFR content and Mtx transport-impairment in T-ALL and B-precursor ALL blasts at relapse. DHFR (top panel) and Mtx transport-impairment (lower panel)were assayed in 8-relapsed B-precursor (designated rBP) and 6 relapsed T-cell (rT) blast specimens by PT430 and flow cytometry. Data are plotted as the relative fluorescence ratios versus patient numbers for one or two separate blast subpopulations (designated S1 and S2) as for Figs 2 and 3. Patient characteristics and percentages for the fluorescent DHFR subpopulations are noted in Table 3. The percentages of the total blasts in relapsed T-cell samples 3 and 6 with impaired MTX transport were 73% and 10%, respectively. As described in Materials and Methods, normal DHFR (fluorescence ratios of 1.6 to 2.4) and Mtx transport (ratio of 1.0 to 1.2) were defined by 5 patients (BP6, 8, 13, 30, and 31 in Table 1) who survived at least 5 years without relapse. Experimentally measured parameters in excess of these ranges were considered elevated or impaired, as appropriate.