The investigations included 333 milk samples obtained from cows of 6 breeds maintained in Poland, i.e. Polish Holstein-Friesian black-white (150) and red-white (40) variety, Simental (29), Whitebacks (28), Jersey (37) and Polish Red (49). Milk was collected in the spring-summer season, when the dietary units included green forage. At the milk sampling a lactation phase was regarded, that is up to 120 days, from 120 to 200 and over 200. In each milk sample there were determined chemical composition, fat molecule diameter, thermostability and rennet induced milk coagulability. The parameters were analysed taking into consideration a cow breed and a lactation phase. It was found that milk of Whitebacks, Simental and Jersey exhibited a better protein-fat ratio and a high level of fat molecules of big-sized diameters. Milk of these cows and Polish Red withstood a sustained thermal treatment at 140°C and showed significantly shorter coagulation time due to a higher protein content. Therefore, milk from cows of these breeds proves more useful for processing, mainly cheese production. The effect of a lactation phase on milk chemical composition, its thermostability and coagulation time was indicated.