A method for the simultaneous quantitation of alpha(S1), alpha(S2), beta, and kappa-caseins in water buffalo (Bubalus bubalis) milk using reverse phase high-performance liquid chromatography was developed. The molecular masses of the peaks separated by the described chromatographic protocol were determined by ESI-MS. alpha(S1)- and kappa-caseins were found to be heteromorphic in several individual milk samples. In particular, alpha(S1)-casein showed two peaks with a molecular mass of 23,490 Da and 23,516 Da, and kappa-casein showed three peaks with molecular masses of 19,165 Da, 19,177 Da, and 19,247 Da. Only one form for beta-casein (24,033 Da) and alpha(S2)-casein (22,741 Da) were detected. The mean values of casein fraction concentration observed throughout the individual samples were 8.89 gL(-1) with a relative standard deviation (RSD) of 20% for alpha(S1)-casein, 5.08 gL(-1) with a RSD of 25% for alpha(S2)-casein, 20.91 gL(-1) with a RSD of 16% for beta-casein, and 4.13 gL(-1) with a RSD of 24% for kappa-casein. Linear and second-order polynomial correlations with total nitrogen were calculated for all casein fractions.