When Swiss 3T3 cells are treated with colcemid for 60 min (0.5-2 micrograms/ml), although the cytoplasmic network of microtubules is considerably reduced, a significant but variable number of microtubules remain. These residual microtubules appear to converge on a single, usually ciliated, organizing centre. In contrast, on removal of the colcemid, microtubule regrowth takes place from two organizing centres in approximately 50% of the cells. Surprisingly, whether a cell has one or two organizing centres during recovery does not depend on its position in the cell cycle, though recovery takes place earlier in serum-stimulated cells than in serum-starved quiescent ones. It seems likely that prior to colcemid treatment, cells contain two potential organizing centres only one of which is normally active, the inactive one becoming active after exposure to the drug.