It is widely known that ceils irradiated with X-rays undergo a slowing of the cell cycle. This occurs at any stage of the irradiated cell cycle, and, in addition to the irradiated stage, always involves the G2 phase. The mitotic delay observed when G2 cells are irradiated has been studied extensively and it has been observed that there is a point in G2 called the transition point (TP) beyond which cells are practically refractory to the induction of mitotic delay (Hodge and Nachtwey, 1972; Highfield and Dewey, 1975). It is particularly interesting that the TP shifts closer to mitosis with increasing X-ray doses (Dewey and Highfield, 1976; Schneiderman et al., 1977; Tomasovic and Dewey, 1978). Thus if a population of cells in G2 is irradiated, only that part which has not gone beyond the TP is appreciably delayed. This fraction increases with increased dose. In recent years there has been a great interest in G2 repair of chromosome aberrations (CA) and lethal effects (Hartley-Asp et al., 1980; Kihlman et al., 1982; Preston, 1980; Andersson, 1982; Hansson et al., 1982; Natarajan et al., 1982). An attempt has been made to determine the relation between repair phenomena and mitotic delay (Painter and Young, 1980; Scott and Zampetti-Bosseler, 1980; Zampetti-Bosseler and Scott, 1981; Ching and Pardee, 1982; Lucke-Huhle, 1982). The present experiments were carried out in human lymphocytes to study X-rayinduced CA in G2 cells that had and had not gone beyond the TP. Since the TP shifts closer to mitosis with increasing doses, by using different doses of radiation it was possible to compare the damage induced in cells which, even though in the same stage at the time of irradiation, do or do not undergo mitotic delay.