HeLa cells irradiated with a single or two split doses of alpha particles or X rays were observed with time-lapse photography or examined for their colony-forming ability. The cell cycle-dependent variation of cell killing and division delay were compared in synchronous and asynchronous cell populations. Cellular damage by alpha particles was manifested in the form of cessation of division, or death, rather than partial division which was predominant for X irradiation. Furthermore, an increased number of interrupted divisions leading to death following split doses correlated with the lack of production of reparable damage from alpha particles. As to the cell cycle-dependent variation of radiosensitivity, a somewhat different pattern was noted with alpha particles, especially for division delay. The pattern of cell killing with alpha particles was similar to that found with X rays, in that high sensitivity was noted at or close to mitosis, while a resistant peak remained at late S but not in early G1. The pattern of division delay was similar for X rays and alpha particles during G2-M, with a maximum delay at mid G2 and no delay past the transition point, but differed during G1-S. During this period, division delay increased with cell age, whereas it showed a broad peak at G1-S boundary and a trough at late S for X rays. There was a reverse correlation between division delay and cell killing except for G2-M in the case of X rays. However, such was not the case for alpha particles.