We have analyzed the interaction of soluble nuclear lamins with the nuclear envelope by microinjection of normal and mutated lamins into the cytoplasm of Xenopus laevis oocytes. Our results demonstrate that the conserved cysteine of the carboxy-terminal tetrapeptide Cys Ala/Ser Ile Met of lamins is essential for their association with the nuclear envelope. Removal of this sequence or replacement of the cysteine by serine resulted in Xenopus lamin L1 remaining in a soluble, non-envelope-associated state within the nucleus. Similar mutations of Xenopus lamin A resulted in only partial reduction of nuclear envelope association, indicating that lamin A contains additional signals that can partially compensate for the lack of the cysteine. Mammalian lamin C lacks this tetrapeptide and is not associated with the nuclear envelope in our experimental system. Cloning of the tetrapeptide Cys Ala Ile Met to the carboxy terminus of human lamin C resulted in lamin being found in a nuclear envelope-associated form in oocytes. Mutations at the amino terminus and in the alpha-helical region of lamin L1 revealed that the carboxy terminus mediates the association of lamins with the nuclear envelope; however, this alone is insufficient for maintenance of a stable association with the nuclear envelope.