The completed sequence of the arenavirus, lymphocytic choriomeningitis virus, revealed a new gene encoding a small protein with a single zinc-binding domain. The cDNA for this gene has been expressed in E. coli to produce fusion protein that has been used to raise antisera. The antisera facilitated the positive identification of the p11 'Z' gene product as a structural component of the virion. A related arenavirus, Tacaribe, has a comparable p11 gene product. The abundance of the p11 Z protein relative to other virion components has been determined by metabolic labeling. Triton X-114 extraction and dimethyl suberimidate-HCl crosslinking indicate that the p11 Z protein is a hydrophobic protein associated with the nucleocapsid of the virion core.