The effects of αs1-, β-, κ-casein and β-lactoglobulin genetic variants on rennet clotting time, rate of curd firming and coagulum strength, measured on the individual milks of 306 Rendena cows by a lactodynamographic apparatus, were evaluated statistically, with simultaneous adjustment for environmental and physicochemical factors. The importance of genetic polymorphism was confirmed, especially for the main effects of casein loci: interaction among them was not found to be significant. For κ-casein, BB genotype showed the best coagulation parameters (lower rennet clotting time, a higher rate of curd firming and higher coagulum strength), with AB in an intermediate position for clotting time and rate of curd firming. However, for αs1-casein and β-casein, dominance and overdominance effects between alleles were observed. Herd by genotype interaction was also found to be significant (herd by β-casein) or close to significance level (herd by κ-casein). From the results of the analyses, suggestions for selection programmes have been made.