In this study, acute phase proteins including haptoglobin, serum amyloid A, fibrinogen, albumin and total protein levels were investigated in cows with Staphylococcus aureus (S. aureus) positive subclinical mastitis. Healthy Jersey cows in lactation constituted the animal materials of the study. CMT test were applied to detect subclinical mastitis. In 21 milk samples collected from cows with subclinical mastitis, S. aureus was identified by PCR after bacteriologic isolation. These milk samples formed the study group. Healthy milks in which no bacteriological growth was observed were used as the control group. Elevations in haptoglobin (Hp) and serum amyloid A (SAA) levels in infected milk group compared to control milk group were statistically determined as p<0,05. Although an increase was observed in fibrinogen (Fb), total protein (TP) and albumin (Alb) levels, the variations were not statistically significant.In conclusion, in infected milks, total protein elevation is an expected result due to the presence of somatic cell and bacteria. In the present study, an increase was determined in total protein level, but it had no statistical significance. However, a significant increase was determined regarding haptoglobin and serum amyloid A in infected milk samples. This suggests that haptoglobin and serum amyloid A are more sensitive indicators in subclinical mastitis and may be beneficial in clinical practice.