The effects of colchicine and cytochalasin B on grey crescent formation in frog (Rana pipiens) and toad (Bufo arenarum) eggs were determined. Colchicine prevented the appearance of the grey crescent, but this inhibition was not due to the absence of an aster. Cytochalasin B did not inhibit grey crescent formation, nor did it inhibit certain activation events such as cortical granule breakdown or cortical contraction. Cytochalasin B caused a detachment of the cortex from the cytoplasm and induced the formation of a morphological grey crescent in non-activated eggs. The results suggest that microtubules may play several roles in grey crescent formation and that a change in the attachment of the cortex to the cytoplasm may also be involved.