Histone H1 accumulates in the nucleus after injection into the cytoplasm of Xenopus oocytes. A proteolytic fragment of 89 amino acids encompassing the carboxy-terminal domain also accumulates in the nucleus. Lysine, alanine and proline compose 84% of this domain. Accumulation is not due solely to the high lysine content since poly-L-lysine does not accumulate in the nucleus when injected into the cytoplasm of Xenopus oocytes. Proteolytic fragments encompassing other domains of the molecule are degraded in the oocyte after injection. In these instances degradation is more rapid in the cytoplasm than in the nucleus giving the false impression of accumulation in the nucleus, an artefact which is likely to confuse other studies of protein migration. Susceptibility to rapid degradation is a dominant feature, thus the globular domain destabilises the contiguous carboxy-terminal domain. The properties of the carboxy-terminal domain of H1 and the possible involvement of the amino acids lysine, proline and alanine in migration are discussed and compared with those of a domain that specifies migration of nucleoplasmin into the oocyte nucleus.