An antiserum to a peptide of the Tacaribe virus Z protein was used to determine whether this small Zn(2+)-binding protein was required for viral RNA synthesis in infected cell extracts. Specific immunodepletion of the extracts invariably reduced genome synthesis to near background levels, but strong effects on mRNA synthesis occurred only early in the infection or when mRNA synthesis was relatively weak. Our results suggest that the Z protein is required for both mRNA and genome synthesis, but in somewhat different manners.