Improvement of udder health through a process of genetic selection is relat
ed 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 fa
ctor in mastitis. Thus, in order to analyse the heritability of a trait and
its correlation with udder health, the present study examined the leucocyt
e populations of uninfected mammary glands, their variability among quarter
s in a particular cow, and the changes that occur during lactation. Each on
e of the 20 cows examined was tested on average 3.06 rimes during lactation
. The somatic cell count (SCC)/ml ranged from 12 000 to 151 000, the coeffi
cients of determination (R-2) 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 fo
r any of the dependent variables including SCC. The effect of the cow trait
was found robe significant for polpmorphonuclear (PMN), macrophage (MO), a
nd 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 w
ith udder health in a future study.