The frequency of spontaneous sister chromatid exchange was studied in normal marrow derived from 38 healthy donors and 40 untreated patients with chronic phase CML. The sister chromatid exchange frequency was significantly lower in the leukemic cells (range, 2.32 +/- 1.31 to 4.76 +/- 2.37 per metaphase; mean, 3.18 +/- 0.49) than in normal marrow (range, 2.36 +/- 1.44 to 5.54 +/- 2.24 per metaphase; mean, 3.92 +/- 0.72). The contraction status of chromosomes was comparable in normal and Ph-positive metaphases. The reduction of sister chromatid exchange in leukemic cells was seen in all chromosome groups. The analysis of cell cycle specific proliferation according to the typical staining patterns of metaphases due to the number of cell cycles during which bromodesoxyuridine was substituted, revealed longer cell cycle times for the leukemic cells.