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This study aimed to evaluate the effects of in vitro‐induced drug resistance on the virulence of Streptococcus. Micro‐dilution method was used to determine the minimal inhibitory concentration (MIC). In vitro‐induced drug resistance was conducted for S. agalactiae (CVCC1886) and S. dysgalactiae (CVCC3701) by gradually increasing the antimicrobial concentration (strains were from IVDC, China). PCR was used to detect the resistance and virulence genes of the strains before and after resistance induction. Colony morphology was observed to compare the physiological and biochemical properties of the strains. A total of 88 clean‐grade Kunming mice (obtained from Inner Mongolia University, Hohhot, China) were used in half of the lethal dose (LD50) test for detecting the changes in virulence of strains. The results showed that S. agalactiae (CVCC1886) and S. dysgalactiae (CVCC3701) developed resistance against seven kinds of antibiotics, respectively. Resistance and virulence genes of CVCC3701 were changed when treated by the Penicillin‐inducing. The growth of the CVCC3701‐PEN was decreased compared to the CVCC3701. Virulence test in mice indicated that the LD50 of CVCC3701 before induction and CVCC3701‐PEN after induction were 5.45 × 106 and 5.82 × 108 CFU/ml, respectively. Compared with the untreated bacteria, the bacterial virulence was reduced 1.1 × 102 times after resistance induction. In conclusion, S. dysgalactiae (CVCC3701) is a susceptible strain of drug resistance to antibiotics, in vitro‐induced drug resistance reduced the virulence of CVCC3701, but the virulence is still existing and also could result in the death of mice. For public health safety, it must be alert to the emergence of drug resistance of Streptococcus in animal production. [ABSTRACT FROM AUTHOR]