The technique of percentage labeled mitoses was used to compare radiation-induced division delay in 9L rat gliosarcoma cells growing as spheroids or as exponential monolayers. The length of delay induced by each of five X-ray doses was determined as the difference between control and irradiated cultures in the time required to reach the half-height of the first peak of labeled mitoses. Spheroid cells were delayed significantly longer than monolayer cells; the slopes of the dose responses were 32 and 13 min/Gy, respectively. Cells in small spheroids (150 micron diameter) were delayed to the same extent as cells in large spheroids (800 micron diameter). Like the contact effect previously observed as enhanced radiation survival of cells grown as spheroids, the increased radiation-induced delay may be a consequence of the growth of cells in three-dimensional contact.