This investigation studied the influence of K-casein A and B variants on the composition, production and for the first time the pilot-scale Cheddar and low moisture part-skim Mozzarella cheesemaking properties of milks from Irish Holstein Friesians. Analysis of 6,007 individual animal milks showed that only 1.98 % of animals had the K-casein BB phenotype. No statistically significant associations (P < 0.05) were observed between K-casein variant and milk yield and composition (Chapter 1). Renneting studies showed that rennet coagulation time was related to K-casein variants as follows: K-casein AA>AB>BB while the rate of curd firming and curd firmness values were such that K-casein AA<AB<BB (Chapters 2 & 5). The moisture adjusted Cheddar and Mozzarella cheese yields for AA and BB variant milks were 92.5 and 100.1, and 91.5 and 102.5 kg cheese/1000 kg milk, respectively. The moisture and protein adjusted Mozzarella cheese yields for K-casein AA, AB and BB variants were 90.2, 93.2 and 95.2 kg cheese/1000 kg milk, respectively (Chapters 2 & 5). Cheddar cheeses made from K-casein BB variant milk had significantly higher (P<0.05) free amino acid levels after 270 days ripening than K-casein AA cheeses. Rheometric analysis during a 270 day ripening period showed that K-casein BB Cheddar cheeses were significantly softer (P<0.05) at 90 and 180 days and had a lower fracture stress at day 180 than AA variant cheeses. K-Casein variant had no significant effect on the sensory characteristics of Cheddar cheese (Chapter 3 & 4). Furthermore, K-casein variant had no significant effect on the proteolysis (nitrogen soluble at pH 4.60 and nitrogen soluble in 5 % phosphotungstic acid), rheological (hardness) or functional characteristics (melt, flow and stretch) of Mozzarella (Chapter 5). Given the above results, it was concluded that K-casein BB variant milk resulted in improved rennet coagulation properties and higher yields of Cheddar and Mozzarella cheese while having no detrimental effects on cheese quality. Furthermore, increasing the frequency of the B variant of K-casein in Irish Holstein Friesian dairy herds should be beneficial in the production of milk for the manufacture of Cheddar and Mozzarella cheese.