Proteins which have been extracted from nuclei can re-enter and accumulate in the nucleus when deposited in the cytoplasm of the cell. This phenomenon has been investigated in two nuclear proteins having widely different properties. The same experimental strategy has been used in both cases, that is, microinjection of proteolytic fragments of these proteins into Xenopus oocytes and observing which of these fragments can accumulate in the nucleus. In the case of nucleoplasmin, a large, pentameric acidic protein, which is the most abundant protein of the X. laevis oocyte nucleus, a small fragment has been isolated which is both necessary and sufficient for accumulation in the oocyte nucleus. In the case of calf thymus histone H1, a small basic protein, a C-terminal fragment of 87 amino acids can accumulate in the oocyte nucleus. The amino acids lysine, proline and alanine constitute 73 of the 87 amino acids. Since the other 14 amino acids are scattered, not clustered, these three amino acids must presumably predominate in any sequence which specifies accumulation of the fragment in the nucleus. By using the expression vector lambda gt 11, cDNA clones of nucleoplasmin have recently been obtained and their properties are described.