As mastitis is one of the most significant causes of health problems in dairy cows and, besides economic, also has other negative effects, the aim of this paper is to study the influence of mastitis on reproductive parameters in cows that have an evident and clinically obvious mastitis. In this study we observed the duration of the “service period“ and the number of inseminations (frequency of abortions) as reproductive parameters, up to the time of the next conception of lactating cows. Our research was carried out on a farm with Holstein-Friesian cows in duration of 15 months with breaks. First, 30 heads of cattle of various age were selected, the number of their lactations from 3 to 6. We took samples from the whole amount of milk twice a month in order to find out the number of somatic cells on the farm. The California Mastitis Test was carried out in all selected cattle. From all of the quarters of selected cows which showed a positive reaction with CMT, we took samples of udder (milk) secretion and sent them to the laboratory in the “Dr. Vaso Butozan” Veterinary Institute to estimate the number of somatic cells. Highly sophisticated equipment Fossomatic FC was used to identify the found microflora for these purposes. In all of the 30 cows a secretion disorder was found in one quarter: in 3 cows – in the front left quarter; in 5 cows - in the front right one; in 12 cows - in the back left one and in 10 cows - in the back right udder quarter. After the appropriate therapy was administered, we took the samples of udder secretion again in order to estimate the number of somatic cells. By this study it was estimated that the calving to conception interval in cows with clinical mastitis was ±76 days and the insemination index 2.6. The calving to conception interval referring to the farm in whole was ±53.2 days and the insemination index 1.96. Clinical mastitis followed by the increase in the number of SC prolongs the calving to conception interval to the significant extent, i.e., increases the insemination index.

            Key words: mastitis, calving to conception interval, somatic cell