Diagnosis of subclinical mastitis is of increasing importance and appropriate detection methods are needed. Both haptoglobin (Hp), an acute phase protein in cattle, as well as lactate dehydrogenase (LDH), an ubiquitous enzyme, can be successfully used to detect clinical mastitis. The present paper describes quantification of Hp and LDH in milk samples from healthy and subclinically diseased udder quarters. Hp was analysed in the laboratory using an ELISA. The activity of LDH was measured in raw milk directly in the milking parlor. Both parameters were suitable to distinguish between sterile samples and bacteriologically positive samples. The ability to differentiate between minor and major pathogens was better for Hp in skim milk than for LDH in raw milk. Hp and somatic cell count (SCC) as well as LDH and SCC were positively correlated (r = 0.8 and r = 0.76, respectively). Subclinical mastitis was defined as follows: SCC > 100 × 10 3 cells/ml and bacteriological positive findings in two out of three weekly samples. Sensitivity and specificity were above 85% for Hp and above 81% for LDH. Using a less rigid classification to define mastitis, i.e. SCC > 200 × 10 3 cells/ml and two out of three weekly samples bacteriologically positive, sensitivity for Hp improved (89%) and remained unchanged for LDH. Both parameters are useful parameters for the diagnosis of subclinical mastitis. LDH activity in raw milk was less sensitive and specific than Hp but the method described herein offers the opportunity to measure LDH activity directly in the milking parlor and might therefore be suitable for on-line system developments.