A total of 555 milk samples was collected from 53 Holstein cows for 1 year. Samples were analysed for N-acetyl neuraminic acid (NANA) content, κ-casein content and somatic cell count. The extent of glycosylation of κ-casein was estimated through the NANA content of κ-casein in each milk sample. The overall mean value for the NANA content was 63·6 ± 20 μg/mg of κ-casein with a range of 23–166 μg/mg. Least squares analysis showed significant (P < 0·01) effects of parity, test day, stage of lactation, phenotype for κ-casein, somatic cell count, and cow within phenotype. The NANA content of κ-casein decreased with increasing parity number and increased significantly when somatic cell count was above 200000 cells/ml. After reaching a minimum value between 2 and 3 months after calving, the NANA content of κ-casein increased during the later part of the lactation. NANA content was higher in the milk of cows phenotyped AB for κ-casein than for cows phenotyped for κ-casein AA, thus suggesting that B variant is more efficiently glycosylated than A variant.