The Arabidopsis gene AINTEGUMENTA (At-ANT) functions in cell proliferation and organ growth. The ANT protein has two copies of the AP2 domains, R1 and R2. Recently, a partial cDNA sequence of the At-ANT homolog in Antirrhinum majus (Am-ANT) was reported (Delgado-Benarroch et al., 2009). Here, we used virus-induced gene silencing (VIGS) to analyze the function of the reported Am-ANT. We then determined the open reading frame (ORF) of Am-ANT and its predicted amino acid sequence. We induced VIGS using the Cucumber mosaic virus vector (CMV-A1) that contained partial sequence of Am-ANT (A1:ANT) and suppressed the level of Am-ANT mRNA and noted for any phenotypic changes. The function of Am-ANT was very similar to that of At-ANT. The A1:ANT-infected Antirrhinum plants had smaller floral organs and leaves, even though cell sizes were unchanged in flowers and larger in leaves. The CMV-based VIGS showed that the isolated Am-ANT gene was indeed functional in cell proliferation and organ growth as observed for At-ANT. In conclusion, we found that the CMV vector had the advantage of systemically infecting A. majus without severe symptoms for functional analysis of A. majus genes.