This study emphasizes the problems encountered in obtaining suitable control levels for comparison with occupational studies of exposure to clastogens. Blood samples taken from 135 individuals (69 females and 66 males) were examined for chromosome aberrations. The data include 15,368 cells scored for chromosome aberrations. The frequencies of chromatid and chromosome breaks, acentric fragments and dicentrics were determined. The frequency of the most common chromosome aberration, chromosome breaks, was 1.1 x 10(-2) and for dicentrics 0.26 x 10(-3). The values obtained were investigated in relation to sex, cigarette smoking habits, diagnostic X-ray exposure and use of antibiotics. In all the parameters, no significant differences were found.