αs-Casein and κ-casein were measured in milk and curd by a microparticle-enhanced nephelometric immunoassay (NEPHELIA®). Specifically designed microspheres were coated with antigen (αs-casein and κ-casein) and then were agglutinated by specific antibodies. The light scattered by the agglutinates was quantified with a nephelometer. Antigen-coated microsphere agglutination was inhibited by free antigen solution, allowing its measurement. Calibration curves for αs-Casein and κ-casein in milk and curd, performed with a low heat milk powder as standard, largely covered the usual concentrations of caseins. Accuracy (average ratios of recovery were 98.7 and 104.3%) and precision (coefficients of variation from 1.9 to 7.4%) assessed the fidelity of the method. NEPHELIA®, applied to casein determination, offers many advantages over the classical methods of milk protein measurement: high dilution of the reagents, no pretreatment of the samples, enhanced sensitivity (few micrograms per liter), short reaction time (1h), and easy use (no washing or phase separation).