DHFR contents of B-precursor and T-cell ALL at diagnosis. DHFR levels were assayed with PT430 and flow cytometry and are plotted as relative fluorescence ratios (ie, nonexchangeable PT430/autofluorescence) versus patient numbers for one to three separate blast subpopulations (designated S1 through S3). Patient characteristics and, for heterogeneous populations, percentages for each fluorescent blast subpopulation are indicated in Tables 1 and 2. As described in Materials and Methods, normal DHFR (fluorescence ratios of 1.6 to 2.4) was defined by 5 patients (BP6, 8, 13, 30, and 31) who survived at least 5 years without relapse. Experimentally measured DHFR values in excess of this range were considered elevated.