Surface antigen one (Sao) is a newly identified protein from the major zoonotic pathogen, Streptococcus suis. In search of functional proteins related to the pathogenesis of Chinese S. suis 2 (SS2), unexpectedly, a variant of Sao protein was obtained. To test its prevalence in S. suis, PCR assay was adopted to address the coding genes systematically. It was found that there are three allelic variants of sao gene, namely sao-S, sao-M, and sao-L based on the different lengths of the genes (approximately 1.5, approximately 1.7, and approximately 2.0 kb, respectively). These differences were determined to be caused by heterogeneity within the number of C-terminal repeat sequences (R), which had been seen as a pathogenicity-related domain in the plant pathogen, Xanthomonas oryzae. Two variants (sao-M and sao-L) were only found in SS2. All three variant proteins were prepared in vitro and their biochemical and biophysical properties were characterized. A soluble form of Sao-M protein was then used as a capture antigen to develop an enzyme-linked immunosorbent assay method to detect antibodies against SS2 in convalescent pig sera. Taken together, the results exhibit the properties of Sao proteins and provide an efficient Sao-M-based method for monitoring SS2 infection.