An immunoassay to quantify a S1-casein (a S1-CN) in milk using an optical biosensor, based on surface plasmon resonance (SPR) measurement, has been developed. The assay consists of a two-step sandwich strategy, with two anti-a S1-CN antibodies directed against each extremity of the molecule. This strategy permits only intact a S1-CN to be quantified and not its degradation products. The calibration curve was obtained using a reference milk powder with a known a S1-CN concentration. Analysis time per sample was less than ten minutes. The antibody-coated surface could be used for more than 150 determinations. Detection limit was established at 0. 87 mg/ml and the intra-and inter-assay variation coefficients were 2. 86 and 5. 31%, respectively. The method was applied to raw milk to quantify intact a S1-CN, with no pretreatment of the sample. An initial analysis of 48 milk samples permitted a S1-CN concentrations ranging from 8. 8 to 12. 06 mg/ml to be obtained.