<p>Transient protein expression is an effective tool to rapidly unravel novel gene functions, such as transcriptional activity of promoters and sub-cellular localization of proteins. However, transient expression is not applicable to some species and varieties because of insufficient expression levels. We recently developed one of the strongest agroinfiltration-based transient protein expression systems for plant cells, termed 'Tsukuba system.' About 4 mg/g fresh weight of protein expression in Nicotiana benthamiana was obtained using this system. The vector pBYR2HS, which contains a geminiviral replication system and a double terminator, can be used in various plant species and varieties, including lettuces, eggplants, tomatoes, hot peppers, and orchids. In this study, we assessed the applicability of the Tsukuba system to several species of legumes, including Lotus japonicus, soybean Glycine max, and common bean Phaseolus vulgaris. The GFP protein was transiently expressed in the seedpods of all examined legume species, however, protein expression in leaves was observed only in P. vulgaris. Taken together, our system is an effective tool to examine gene function rapidly in several legume species based on transient protein expression.</p>