We describe an embryonic lethal mutation in Xenopus laevis that provokes regression of cleavage furrow formation. The mutant females (designated as af) were obtained by the back-cross of a female with one of her sons. All the fertilized eggs laid by the mutant females, regardless of the wild-type male used in the mating, failed to cleave although each furrow ran at a proper position superficially. Light and electron microscopic observations of the embryos revealed that the cleavage furrows stayed on the surface and cytoplasmic divisions did not take place at all, while nuclear divisions did. Two-dimensional gel-electrophoretic comparisons of af and wild-type embryos demonstrated that two proteins, having estimated molecular masses of about 38 kDa (pI 6.6) and 78 kDa (pI 7.6), were missing in af embryos. Microinjection of clear cytoplasm from a wild-type egg into fertilized af eggs provoked partial surface contraction and cleavage furrow formation in recipient af eggs. The results showed that the af females carry a lethal maternal-effect mutation which causes cleavage furrow regression by being deficient in a few proteins, and that cytoplasm of wild-type eggs can partially rescue the cleavage furrow formation of af eggs by furnishing the corrective material, presumably a product of the normal allele of af.