The detailed composition, pH, and the properties of rennet clotting, coagulum development and syneresis at pH 6·4 were estimated in milks from 4 Friesian cows during the first 9 weeks of lactation. The concentrations of casein, fat, citrate, Ca, Mg, Pi and Na decreased significantly during early lactation whilst the pH and concentration of lactose increased. Levels of whey protein and K and the casein:fat ratio were unaltered. Considerable variation occurred in the relative proportions of α-, β- and κ-casein. These changes in composition were associated with decreasing coagulum strength and an increased rate of whey drainage although rennet clotting time (RCT) did not follow a significant trend. RCT was positively related to levels of Na and whey protein but negatively correlated with Pi, K and lactose. There were significant positive correlations between coagulum strength and casein, fat, non-protein nitrogen, Pi, Ca, Mg and citrate. The correlation with α-casein was stronger than that with β- or κ-casein. Syneresis time was positively related to fat, Ca and coagulum strength, but negatively related to the concentration of lactose.