Nucleophosmin/B23 is a nucleolar phosphoprotein which forms oligomers. To determine the domain essential for oligomer formation, various deletion and point mutation clones of nucleophosmin/B23 were constructed. Nucleophosmin/B23 and the mutant proteins were produced by (a) coupled in vitro transcription and translation and (b) expression in Escherichia coli with T7 RNA polymerase expression vector (pET-8c). Nucleophosmin/B23 synthesized in vitro has the same peptide map as that synthesized in HeLa cells. Similarly, it formed oligomers which could be detected in SDS/PAGE and were cross-linked with nitrogen mustard in vivo. Substitution of Met5, Met7, and Met9 with Leu or deletion of five amino acids at the C-terminus abolished the oligomerization. Deletion of portions of amino acids in the middle of the molecule (amino acid residues 83-152, 117-186 and 185-240) had little effect on the oligomerization. Co-expression of the N- and C-terminal mutant clones in vitro did not produce oligomers. These results indicate that intra-molecular interactions with both the N- and C-terminal domains are essential for oligomer formation.