A highly specific antiserum was prepared against purified cauliflower mosaic virus viroplasm-protein (VmP). A virus specific in vitro major translation product (TPmaj), encoded by the 19S poly(A) RNA fraction from cauliflower mosaic virus infected turnip leaves, was recognized by this antiserum. The N-terminal sequence of TPmaj corresponds to the sequence following the first in-phase initiation codon in gene VI of the cauliflower mosaic virus genome. Both VmP and TPmaj have blocked termini and probably start from the same AUG codon.