Milks were prepared at 1·7- to 4-fold the initial concentration by combining skim-milk concentrated by ultrafiltration with cream, and used for Cheddar cheese-making. Starter growth was unaffected, but the increased buffering capacity in the more concentrated milks resulted in a slower decline in pH and a higher pH value in the cheese. Curd formation was faster despite the use of reduced amounts of rennet. With milk concentrated more than 2-fold, large amounts of fat were lost in the whey, so that the cheeses had less fat than normally. Fat losses may be partly related to the lower degree of aggregation of the casein micelles when the curd was cut. As the concentration factor of the milk increased, the rate of casein breakdown, the intensity of Cheddar flavour, and the levels of H2S and methanethiol in the cheese decreased. These factors may relate to the reduced concentration of active rennet retained in the curd at pressing.