Chikungunya virus (CHIKV)， a mosquito-borne pathogen， consists of three genotypes: East/Central/South African (ECSA)， West African (WA)， and Asian. Although a current rapid immunochromatographic (IC) test detecting CHIKV E1-antigen showed high sensitivity to ECSA-genotype viruses， it showed poor performance against the Asian-genotype virus that is spreading in the American continents. To understand the basis for the low performance of this IC test against Asian-genotype virus， we re-examined the anti-CHIKV monoclonal antibodies (mAbs) used in the assay for their interaction with E1-antigen of the three CHIKV genotypes. We found that the reactivity of one mAb for Asian-genotype virus was lower than that for ECSA virus. Comparison of E1 amino acid sequences revealed that the ECSA virus used to generate these mAbs possesses glutamic acid (E) at position 350， in contrast to WA and Asian， which possess aspartic acid (D) at this position. Site-directed mutagenesis confirmed that the mutation altered mAb reactivity， since E-to-D substitution at position 350 in ECSA reduced recognition by the mAb， while D-to-E substitution at this position in Asian and WA increased affinity for the mAb. Taken together， these results indicate that residue 350 of the CHIKV 6K-E1 is a key element affecting the performance of this IC assay.