The 3' proximal open reading frame (ORF 11) in the citrus tristeza virus (CTV) genome potentially encodes a protein of 209 amino acids with an estimated molecular weight of 23 kDa (p23). The p23 ORF from the severe Florida strain T36 of CTV was expressed in Escherichia coli, and the expressed protein was used to raise polyclonal antibodies in a rabbit. Using these antisera on a Western blot, a protein of expected size (23 kDa) was detected in tissue extracts from CTV-infected citrus but not from uninfected citrus. Most of the p23 protein was found in the soluble, cytoplasmic fraction. Comparison of the sequence of p23 genes from several biologically and geographically diverse CTV isolates indicated a high degree of conservation for this gene and for the RNA binding motif in particular. A cluster dendrogram of the deduced amino acid sequences correlated with the biological properties of the isolates, forming distinct groups of mild, quick decline on stem pitting-inducing isolates. Therefore it is possible that, in addition to the capsid protein gene, the p23 gene also may serve as an indicator for disease severity.