Open reading frame (ORF) V of cauliflower mosaic virus (CaMV), the candidate for the reverse transcriptase gene, has been expressed in E. coli under control of the PR promoter of bacteriophage lambda either as an N-terminal polypeptide fused to beta-galactosidase or as the total ORF V without fusion. Antibodies against these proteins were used to analyze extracts from CaMV-infected plants by immunoblotting. ORF V-specific polypeptides of 80, 62, 58, 22, and 18 kD apparent molecular weights were detected, with the largest species corresponding to the full length translation product. The 62 and 22 kD species could be assigned to the N-terminus and the remaining two species to the C-terminus of the ORF.