Human peripheral blood lymphocytes (PBL) maintained in vitro for 72 hours were labeled with 5-Bromodeoxyuridine (BrdU) at 50 ug/ml and concomitantly treated with girostan at 1.5; 2 and 2.5 ug/ml during the last 13; 11; 9; 7; 5 and 3 hours before cells gathering to estimate the chromosome aberrations induced during different stages of cell cycle traverse as determined by incorporation banding pattern observed in metaphase spreads; similar untreated cultures but labeled with BrdU were set-up as controls in order to determine the phase of cell cycle traverse in the moment when girostan was given to the cultures. Chromosome aberrations were induced especially when treatments were performed 13 and 11 hours before fixation. Their frequency was rather low (up to 7% of observed metaphases) and the lesions were not dose-related. When cells were treated during S- and G2-phases, chromatid and chromosome gaps, breaks, deletions and fragments were observed. Chromatid rearrangements as tri- or quadriradials were seen only when girostan was given during middle S-phase indicating that DNA breaking and misrejoining are successive steps during the same replication round. When cells were exposed to BrdU and girostan during the last 3 hours of incubation, no labeled metaphase or structural aberration was seen, but spiralization defects and premature separation of despiralated chromatids frequently appeared. This could indicate that the induction of chromosome condensing factor and of proteins responsible for normal centromere activity is disturbed when girostan treatment is applied during the last part of G2-phase, in the immediate proximity of mitosis.