Studied was the milk productivity (milk yield and butterfat percent) of 255 normal and 255 diseased cows with subclinical mastitis, both groups pertaining to the Bulgarian Brown breed within the region of the Sofia Pedigree Breeding Inspection. All animals were examined at one and the same time during the period of their third lactation, and the milk productivity data obtained referred to the same period. It was established that there were no statistically significant variations between milk productivity data of the healthy cows and those of the cows with subclinical mastitis for a 300 day period. The relative number of cows with subclinical mastitis showing a milk yield of up to 4000 kg was lower as compared with the same number of the negatively reacting cows. On the other hand, cows showed yields of more than 4000 kg milk manifested reverse trends.