A differential allele-specific accumulation of κ-casein mRNA that is not linked to the κ-casein protein variants is described in Holstein cows. Actually, cows genotyped κ-casein AB were a mixed population. For the first group of κ-casein AB cows, allele A-specific κ-casein mRNA contents within mammary epithelial cells were lower than the allele B-specific ones (cows LH), suggesting that the allele A-specific κ-casein gene was expressed with lower efficiency in mRNA. For the other group of κ-casein AB cows, allele A- and B-specific κ-casein mRNA accumulated to a similar level within mammary epithelial cells (cows HH). The objective of this study was to determine whether the accumulation of allele-specific κ-casein mRNA remained constant throughout lactation for the two groups of cows. Quantitative RT-PCR was used to monitor Holstein cows κ-casein AB genotyped HH and LH throughout lactation for the proportion of allele B-specific mRNA accumulation relative to the total κ-casein encoded mRNA within mammary epithelial cells: RNA was extracted from milk somatic cells known to contain a small proportion of mammary epithelial cells. Mean values of allele B-specific mRNA content were 50·6±0·5 and 54·0±0·9%, for cows HH and cows LH, respectively, and did not vary during lactation (P>0·10). This suggests that the phenotypic expression of the genetic mutation that causes the differential allele-specific accumulation of κ-casein mRNA was not affected by physiological and environmental factors, which tend to vary considerably throughout lactation.