We have cloned and characterised a cDNA encoding Z protein of recently identified MX strain of lymphocytic choriomeningitis virus (LCMV) persistently infecting human MaTu cells. Deduced amino acid sequence of LCMV MX Z protein showed 88.9% identity with that of the LCMV Armstrong (ARM) strain and 80.9% identity with that of the LCMV Traub (TRA) strain. It contained conserved zinc-binding RING finger domain and C-terminal proline-rich region. Northern blot analysis of total RNA from MaTu cells revealed presence of abundant truncated forms of L RNA. Z protein-specific rabbit antibodies were produced to glutathione S-transferase (GST)-Z fusion protein expressed in E. coli and used for the detection of Z protein in MaTu cells. Western blot and immunofluorescence analyses detected relatively high levels of Z protein indicating its role in maintenance of persistent LCMV.