Nucleoplasmin is the most abundant protein of the nucleus of Xenopus laevis oocytes. It rapidly enters the nucleus after being injected into oocyte cytoplasm. Partial proteolysis of the nucleoplasmin pentamer reveals two structural domains within each subunit: a relatively exposed "tail" and a protected "core." When all the "tails" have been removed from the pentamer the residual "core" remains pentameric. This pentameric core fails to enter the nucleus, remaining stably in the cytoplasm. A single tail region attached to the pentamer is sufficient to transport it into the nucleus. The rate of accumulation in the nucleus, but not its final extent, depends on the number of tails per pentamer. The detached (monomeric) tails rapidly accumulate in the oocyte nucleus, indicating that the tail structure is sufficient for selective accumulation. Pentameric cores diffuse throughout the nucleus but are retained when microinjected into the nucleus, indicating that the tail is necessary for entry but not for retention within the nucleus. An improved method for purification of nucleoplasmin is also described.