Abstract: 21 clinical healthy Holstein dairy cows （CMT-negative）, 19 dairy cows infected with recessive mastiffs （CMT -positive）and 33 dairy cows infected with clinical mastitis were randomly selected, and blood samples were taken from the jugular vein for measuring serum biochemical indexes of total protein （TP）, albumin （ALB）, globulin （GLOB）, alkaline phosphatase （ALP）, N-acetyl-β-D- glucosaminidase （NAG）and immunoglobulin IgA, IgM and IgG. The resuhs showed that the serum ALP and NAG activities of the dairy cows infected with clinical mastiffs were significantly higher than those of healthy dairy cows and the dairy cows infected with recessive mastitis （ P〈0.01 or P〈0.05 ）. the serum immunoglobuliin IgA, IgM and IgG contents of the coinical dairy cows were significantly higher than thoseof the healthy dairy cows（P〈0.01 or P〈0.05 ）. The serum ALP and NAG activities and immunoglobulin IgA, IgM, IgG contents of the dairy cows infected with recessive masfitis were higher than those of the healthy dairy cows, but there had no significant difference （P〉0.05）. The serum TP, ALB and GLOB contents among 3 groups of dairy cows were not significant difference （P〉 0.05 ）.