The product of the gene RCC1 (regulator of chromosome condensation) in a BHK cell line is involved in the control of mitotic events. Homologous genes have been found in Xenopus, Drosophila and yeast. A human genomic DNA fragment and complementary DNA that complement a temperature-sensitive mutation of RCC1 in BHK21 cells encode a protein of relative molecular mass 45,000 (Mr 45K) which is located in the nucleus and binds to chromatin. We have recently isolated a protein from HeLa cells that strongly binds an anti-RCC1 antibody and has the same molecular mass, DNA-binding properties, and amino-acid sequence as the 205 residues already identified. HeLa cell RCC1 is complexed to a protein of Mr 25K. We have shown that this 25K protein has a sequence homologous to the translated reading frame of TC4, a cDNA found by screening a human teratocarcinoma cDNA library with oligonucleotides coding for a ras consensus sequence, and that the protein binds GDP and GTP. We have referred to this protein as the Ran protein (ras-related nuclear protein). In addition to the fraction of Ran protein complexed to RCC1, a 25-fold molar excess of the protein over RCC1 was found in the nucleoplasm of HeLa cells. Here we show that RCC1 specifically catalyses the exchange of guanine nucleotides on the Ran protein but not on the protein c-Ha-ras p21 (p21ras).