The aim of the reported study was to investigate the reproducibility of the single-cell gel electrophoresis (SCGE) assay in the determination of DNA singlestrand breaks (SS Bs) and to estimate the statistical requirements when the SCGE assay is used for the detection of genotoxicity in humans In human peripheral mononuclear leukocytes (PM Ls), we repeatedly measured the rate of SS Bs after in vitro incubation of cells for 1 h at 4 °C in phosphate buffered saline (PBS, basal) or 10 FM or 50 p M H 202 (induced) Intra-assay variation was determined from cryopreserved PM Ls of a single donor To assess intrasubject and intersubject variation, PM Ls of ten healthy, nonsmoking subjects (aged 19-37 years) were tested 5-9 times Cryopreserved cells revealed a mean coefficient of variation of 18 % (PBS) and 7 %-9 % (H 2 0 2 ) There were statistically significant differences between individuals in the rate of SS Bs after incubation in PBS (P < 0 01), 10 .M H 2 0 2 (P < 0 001), and 50 t M H 2 0 2 (P < 0 001) The range of interindividual variability was 26 % for basal and 12 %-13 % for induced SS Bs, and the coefficient of intraindividual variation was 18 %-72 % (PBS) and 7 %-23 % (H 2 0 2 ) Neither basal nor induced rates of DNA damage were related to gender or age Estimates of the minimum detectable effects were based on these observed sources of variability (power 90 %, level of significance 5 %, assumed sample size 50) With two different groups, a difference of 31 % in basal SS Bs or 12 % in induced SS Bs would be detectable Repeated measurement within one group could detect a difference of 26 % in basal and 9 % in induced SS Bs In summary, the SCGE assay appears to be suitable for the detection of single-strand breaks, e g , in biomonitoring or environmental medicine, and the statis-This work is part of a thesis by A Kastner for a medical doctorate at the University of Hamburg. tical requirements could be derived from our analysis of the sources of variability.