na Methods Cultures of peripheral blood lymphocytes were made using a modification of the method of Moorhead et al (15). It had been determined from previous attempts of leukocyte cultures in these people that the chances of successful cultures were greatly enhanced if most of the plasma above the buffy coat in the settled blood sample was discarded and only a few ml of plasma used for culture along with the buffy coat. Cultures were harvested at 48 and 72 hours. The slides were stained with aceto-orcein and examined with phase microscopy. Intact and well-spread cells in meta- phase were selected by scanning the slides with low-power magnification. Each cell thus selected was then examined with oil immersion. Examination was begun by comparing aberration rates seen in 48-hour and 72-hour cultures from 5 subjects chosen at random. No significant differences were found. Thereafter, with the exception of one case, all other examinations were made on 50 cells of each individual from the 72-hour cultures only. Thus, a total of 2150 cells was examined in the 43 exposed and 400 cells in the 8 comparison individuals. Aberrations were scored as follows: 1) aneuploidy, including polyploid cells, 2) chromosome aberrations, and 3) chromatid aberrations. Karyotypes were made in cases where the counts were equivocal or where chromosomes of questionable morphology were seen. Only those aberrations were included in the final tabulation that were agreed upon by four observers. Additional karyotypes were made from eight euploid cells showing no gross abnormalities from each of four subjects who showed more aberrations than most others. This was done in an effort to detect in a small sample of this population minor structural abnormalities such as small deletions or inversions that could easily escape notice on microscopic examination. Finally, bone marrow preparations were made from two exposed persons and one control. Only one preparation from an exposed subject contained enough suitable cells in metaphase to warrant evaluation. Results As seen in Table I, aneuploid cells ranged from 5.5 per cent in the unexposed people to 10 per cent in the high exposure group, the low exposure group falling between the two with 8 per cent. In the majority of cells aneuploidy was due to loss of chromosomes. Although the percentage of aneuploid cells in the high exposure group was almost twice that seen in the comparison group, an incidence of 10 per cent may be considered at the upper limits of normal. Polyploid cells were seen with about equal frequency in all three groups. The number of chromatid gaps and breaks and of isochromatid gaps were 2.75 per cent in the unexposed, 3.86 per cent in the high exposure and 5.31 per cent in the low exposure group. These differences were not significant. 138