Reversible acetylation of core histones plays an important regulatory role in transcription and replication of chromatin. The acetylation status of chromatin is determined by the equilibrium between activities of histone acetyltransferases (HATs) and histone deacetylases (HDACs). The Xenopus protein HDACm shows sequence homology to other putative histone deacetylases, but its mRNA is expressed only during early development. Both HDACm protein and acetylated non-chromosomal histones are accumulated in developing oocytes, indicating that the key components for histone deposition into new chromatin during blastula formation are in place by the end of oogenesis. Here we show that the 57 kDa HDACm protein undergoes steady accumulation in the nucleus, where it is organized in a multiprotein complex of approx. 300 kDa. A second, major component of the nuclear complex is the retinoblastoma-associated protein p48 (RbAp48/46), which may be used as an adaptor to contact acetylated histones in newly assembled chromatin. The nuclear complex has HDAC activity that is sensitive to trichostatin A, zinc ions and phosphatase treatment. The 57 kDa protein serves as a marker for total HDAC activity throughout oogenesis and early embryogenesis. The active HDACm complex and its acetylated histone substrates appear to be kept apart until after chromatin assembly has taken place. However, recombinant HDACm, injected into the cytoplasm of oocytes, not only is translocated to the nucleus, but also is free to interact with the endogenous chromatin.