The size of single-stranded DNA in lymphocytes in GO stage from 22 patients with acquired primary and secondary aplastic anemia was estimated by alkaline sucrose gradient centrifugation. The average size was 9.3 (+/-0.3) times 10(7) daltons. The lymphocytes of patients contained significantly more single-strand breaks in DNA, compared to those of normal persons. The difference in size of single-stranded DNA which had been present in nontransformed lymphocytes could also be observed in transformed lymphocytes. Some characteristic differences could be observed in the sedimentation patterns of single-stranded DNA in the lymphocytes of patients with aplastic anemia and those of normal persons. The single-strand breaks in DNA suggested that the repair processes were disturbed in the DNA molecules of circulating lymphocytes from patients with acquired primary and secondary aplastic anemia.