Two commercially available pasteurizers for on farm pasteurization of milk intended for feeding calves were tested for their efficiency to inactivate mastitis pathogens. Raw bulk tank milk of the experimental farm Schaedtbek of the Max Rubner-Institute was artificially contaminated with twelve different strains of mastitis pathogens (intended level 7-8 log10 colony forming units [cfu]/ml). The average contamination level was 7.6 log10 cfu/ml in trials with pasteurizer 1 (P1) and 7.3 log10 cfu/ml in trials with pasteurizer 2 (P2), with lowest counts for yeasts (5.1 log10 cfu/ml for P1 and P2). Average reduction rates of > 5.8 log10 cfu/ml for P1 (72 degrees C, 12 s) and > 6.2 log10 cfu/ml for P2 (64 degrees C, 35 min) revealed an appropriate efficiency of both pasteurizers for practical purposes. Pathogens surviving pasteurization (enterococci in trials with both pasteurizers and of Staphylococcus aureus and Escherichia coli with P1) demonstrate the limits of pasteurization as a function of raw milk quality and emphasize the necessity for appropriate handling of pasteurized milk to prevent excessive multiplication of microbial pathogens.