For DNA strand break analysis in human white 10 persons without any apparent genotoxic expoblood cells, usually metrizoate-Ficoll centrifugation sure. A mean normalized elution rate of 1.30 { is used to isolate mononuclear cells. This procedure 0.38 (95% confidence interval) was detected in isois time-consuming and requires at least 20 ml of lated mononuclear blood cells, and a similar mean blood per sample. Therefore, we developed a tech-normalized elution rate of 1.41 { 0.50 was obnique which does not require isolation of white tained using the direct method. The difference was blood cells prior to DNA strand break analysis by not statistically significant. Five patients treated with alkaline elution (direct method). The sensitivity of a combination chemotherapy consisting of cyclothis new technique was compared to that of the phosphamide (750 mg/m 2 i.v.), doxorubicin (50 standard method, which includes isolation of mono-mg/m 2 i.v.), vincristine (1.4 mg/m 2 i.v.), and nuclear blood cells. A statistically significant in-prednisolone (100 mg/m 2 p.o.) for non-Hodgkin's crease in sensitivity was observed using the direct disease were analyzed for DNA single-strand method. After in vitro gamma-irradiation of venous breaks before and 16 -18 hr after the application blood, an increase in the elution rate of 7.7 1 10 03 of chemotherapy. Increases in mean elution rate hr 01 /Gy was detected if mononuclear blood cells of 68% and 116% were detected using the stanwere isolated compared to 10.5 1 10 03 hr 01 /Gy dard and the direct methods, respectively. For the with the new technique (P õ 0.05). Incubation of direct method, only 3 ml of venous blood were venous blood with ethylene oxide for 1 hr caused sufficient for analysis of one sample, compared to an increase in the elution rate of 5.8 1 10 03 hr 01 / 25 ml needed if mononuclear cells were isolated, mM ethylene oxide for the standard and 12 1 10 03 and about 4 hr of work per assay can be saved . h 01 /mM for the direct method (P õ 0.05). DNA Environ. Mol. Mutagen. 29:58-62, 1997 single-strand breaks were detected in blood cells of ᭧ 1997 Wiley-Liss, Inc.