We have initiated a series of experiments to quantify the impact of environmental variables on the observed frequency of micronuclei in monolayer cultures. In this paper the influence of preirradiation of cell culture vessels on micronucleus formation in Chinese hamster ovary cells was examined. Dry cell culture vessels were preirradiated with 2 Gy of either alpha particles or X rays and immediately plated with nonirradiated cells. About 48 h later a group of randomly chosen containers was set aside, and the rest of the containers were exposed to a range of doses of X rays or alpha-particle radiation. Nonirradiated cells plated on previously irradiated cell culture surfaces manifested nearly as many micronuclei as the irradiated cells. In all experiments, preirradiation of the cell substrate (the culture dish) led to a significantly increased micronucleus frequency relative to unirradiated substrate. These results suggest that methods of cell culture vessel sterilization and the composition of cell attachment surfaces could be a confounding factor, particularly in low-dose experiments.