In order to assess the potential of cytogenetic determinations on peripheral blood lymphocytes as a mean of monitoring human population subjects to occupational and environmental exposures to genotoxins, accurate baseline data are required. During the past 20 years many results of the cytogenetic studies on peripheral blood lymphocytes from monitored occupationally exposed and non-exposed groups were obtained. At the time of blood drawing a questionnaire was administered. The questions covered a brief medical and family history including age, sex, medication, infectious diseases, smoking habits, X-ray examinations, alcohol consumption etc. Cytogenetic analysis from whole blood was carried out in short-term cultures. The cultivation time was 52 hours with all cells being in the first mitosis. A total of 100 well-spread metaphases containing 46 +/- 1 centromere were examined per donor on coded slides. Four categories of chromosome aberrations were evaluated: Chromatid and chromosome breaks, chromatid and chromosome exchanges. Cells bearing breaks or exchanges were classified as aberrant cells. Gaps were recorded but not scored as aberrations. Results of the cytogenetic analysis from control individuals (N = 5,430) indicated elevation of spontaneous frequency of aberrant cells (AB.C.) with age. We found 1.10% AB.C. (N = 551) in newborns; 0.71% AB.C. (N = 105) in the group 5-6 yr; 1.20% (N = 1,734) in the group 7-15 yr; 1.25% AB.C. (N = 239) in the group 16-19 yr and 1.59% (N = 2,801) in the group 20-63 yr.