Coverslip cultures of mammalian cells in logarithmic growth were exposed to 200, 400, and 600 r x radiation and fixed at 12-hr intervals up to 108 hr after exposure. The generation time of untreated cells was about 24 hr. Mitotic figures were scored and classed according to their number of poles at different intervals after irradiation. The relative number of mitoses with a high number of poles increased steadily with time, suggesting that multipolarity was a secondary consequence of radioinduced blockage of division rather than a primary effect of x radiation on duplication of the centrioles. The shift in the relative proportion of high and low multipolar spindles was more pronounced at the highest dose, where division hindrance was more complete. (C.H.)