Argon ion laser irradiation of L929 cells transiently inhibits both entry into and passage through mitosis without affecting clonogenic survival. Anaphase mitotic figures virtually disappear from irradiated cell monolayers although prophase + metaphase mitotic figures can still be identified. The total number of mitotic figures does not change significantly and time-lapse video recording shows that cells do not enter mitosis following irradiation. This effect is dependent on light dose within the 900-2700 J/cm2 range and persists for 10-48 h depending on the initial light exposure. Inhibition of cell locomotion and subsequent recovery were observed to occur over a similar time course. The possible contribution of these phenomena must be considered whenever biological systems are exposed to argon ion laser irradiation.