The aim of the study was to investigate the effect of Ascophyllum nodosum, an edible brown macroalga, on some haematologic parameters of dairy cows. Nineteen clinically healthy Holstein cows, an average 4.3 years old, were divided into two groups for 49 days. Ten cows received control diet (roughages and concentrate) while the concentrate of 9 cows was additionally supplemented with 80 g A. nodosum/cow/day. Average daily milk production (controls 39.6 kg/cow; A. nodosum 40.2 kg/cow), milk protein and fat were not affected by the alga supplementation. Glucose, sorbitol dehydrogenase, haemoglobin, haematocrit and white/red blood cells were evaluated in weekly blood samples. It was seen that A. nodosum increased blood glucose and decreased sorbitol dehydrogenase compared to controls, without any adverse effects on the other examined parameters. Consequently, A. nodosum may be suggested as a functional ingredient in dairy cow nutrition, improving energy utilization and expressing hepatoprotective effect.