Tacaribe arenavirus S RNA was cloned and analysis of its nucleotide sequence revealed two open reading frames of significant size, one in the virus-sense strand, the other in the virus-complementary strand. The predicted amino acid sequences of the two reading frames were compared with the predicted primary structures of the nucleoprotein (N) and glycoprotein precursor (GPC) of LCM, Pichinde and Lassa viruses. The results indicated a high degree of homology between the proteins of similar properties. It was also found that in Tacaribe virus-infected cells a subgenomic viral-sense GPC RNA and a subgenomic viral-complementary N RNA are synthesized in addition to the full length viral (v) RNA and viral complementary (vc) RNAs. These results support the conclusion that in Tacaribe virus--as in Pichinde and lymphocytic choriomeningitis arenavirus-S RNA encodes the viral N and GPC proteins and has an 'ambisense' coding strategy. Analysis of the S-derived RNA species at early times post-infection in cells incubated with or without inhibitors of protein synthesis indicated that for primary transcription of the N mRNA, protein synthesis is not required; whereas synthesis of the vc RNA, GPC mRNA and v RNA does require protein synthesis to take place.