Glycophorin A (GPA) assays for human erythrocytes with gene expression loss and duplication phenotypes (NO, NN) were carried out on 15 Chernobyl clean-up workers (liquidators) who immigrated to Israel within the preceding 5 years, 19 local Israeli controls, and 14 Russian (nonliquidator) immigrants. GPA phenotype variants in red blood cells of the 15 liquidators showed values ranging from 1 to 101 events/10(6) cells, with a mean +/- SD of 25.6 +/- 7.0. In comparison, the 19 Israeli controls had values ranging from 0 to 13 GPA events per 10(6) cells, with a mean +/- SD of 3.9 +/- 0.8. The difference was highly significant (p < 0.001). Another group of 14 volunteer control subjects (nonliquidators) who had emigrated from the former Soviet Union to Israel during the past 5 years showed values ranging from 0.0 to 35.0 events per 10(6) cells, with a mean +/- SD of 6.1 +/- 2.7. The difference between this group and the liquidator group was significant at p < 0.01. The results are compatible with past exposure to radiation in the group identified as liquidators.