Abstract: The present study was aimed to establish a method for quickly and accurately calculating the concentration of Streptococcus agalactiae antigen in dairy cow mastitis vaccine. Streptococcus agalactiae were washed from blood plate using physiological saline and diluted into different concentrations of bacteria suspension，which were used to calculate the number of viable bacteria suspension by plate count method.Meanwhile the corresponding absorbance （A_{450 nm}） were determined. Then established the standard curve through the correlation between concentrations and absorbance.The result showed that the viable bacteria counts had a significant linear correlation with the absorbance of Streptococcus agalactiae.Thus，we build a regression equation between Streptococcus agalactiae bacteria suspension concentration and absorbance was y＝7.5861x－0.0805，R²＝0.9735，through which we could quickly calculate Streptococcus agalactiae concentration by measure the absorbance of its suspension.Compared to traditional plate count method，the absorbance measurement accuracy of the method was verified better by 5 batches of Streptococcus agalactiae bacteria suspension fostered by fermentor，and proved the new method has characteristics of simple，fast and accurate.

Key words:

bovine mastitis; Streptococcus agalactiae; bacterial count; absorbance; relations