The aim of this paper is to develop a method for determining the typical chemical composition of the milk of Polish Holstein-Friesian (PHF) cows. The paper uses data collected from 1329 test-day milking records from 20 herds of PHF dairy cattle in Sokołów County, from 2009 to 2015. The effect of the following factors on the chemical composition of milk was determined: lactation stage (15 one-month stages); age of cows (lactations 1, 2, 3–4, and 5–7); genotype (share of PHF breed: less than 50%, 50–75%, 75–82.5% and more than 82.5%); somatic cell count (SCC) in 1 ml of milk (in thousands: 0–200, 200–400, 400–1000 and more than 1000); feeding level (fat to protein (F/P) ratio): ≤1.0, 1.0–1.4, 1.4–1.7 and >1.7); calving season (autumn/winter, spring/summer) and daily milk yield (milk yield in kg: ≤15, 15–25, 25–35 and >35). Nutrition and udder health status were found to be the main factors influencing the chemical composition of milk. For selected cows with optimally balanced feed rations (F/P ratio in milk from 1.1 to 1.4) and a low somatic cell count (SCC ≤200,000/ml), daily yield was the main factor affecting the chemical composition of the milk. It was also concluded that government and scientific publications on the PHF breed should take into account the impact of the F/P ratio, SCC and yield of milk on its composition.