Impact of milk protein composition on casein (CN) retention in curd during the milk coagulation process was studied using a model cheese making system. Individual milk samples from 110 cows in mid lactation of the Swedish Red and Swedish Holstein breeds with known genotypes of β-casein, κ-casein and β-lactoglobulin were defatted, coagulated with chymosin, subjected to syneresis and subsequent pressing simulated by centrifugation. The results indicated that κ-casein concentration of milk plays an important role in the curd formation process and initial syneresis (whey after cutting), whereas an increased CN ratio was associated with less casein in whey after simulated pressing. Increased κ-casein concentration of milk also characterized the milk samples with no measurable loss of casein in whey, compared with milk samples with casein lost in whey, both after cutting and after simulated pressing. Concentrations of αs1-casein, β-casein, and total casein in milk were positively associated with fresh curd yield, which showed a strong correlation with amount of casein retained in curd. No effect of protein genotype on fresh curd yield or casein in whey was found. The β-lactoglobulin BB genotype was associated with increased casein retention in curd, most likely due to the association of this genotype with CN ratio.