Improvement of udder health through a process of genetic selection is related to heritability and the role of the specific trait in the probability of an individual cow developing an infection. It was suggested that different patterns of leucocyte population of the healthy gland are a significant factor in mastitis. Thus, in order to analyse the heritability of a trait and its correlation with udder health, the present study examined the leucocyte populations of uninfected mammary glands, their variability among quarters in a particular cow, and the changes that occur during lactation. Each one of the 20 cows examined was tested on average 3.06 times during lactation. The somatic cell count (SCC)/ml ranged from 12,000 to 151,000, the coefficients of determination (R2) were higher than 0.5 for SCC. No significant differences were found in the dependent variables between the sampled times (test) nor any interaction between the slopes calculated for the cows over time. No significant differences were found among quarters within a cow for any of the dependent variables including SCC. The effect of the cow trait was found to be significant for polymorphonuclear (PMN), macrophage (MO), and T-lymphocyte-bearing CD4+. The number of lymphocytes labelled with the anti-B monoclonal antibodies was negligible. In conclusion the patterns of leucocyte populations in milk together with the variance among cows should enable an analysis of the heritability of this trait and its correlation with udder health in a future study.