The public concern surrounding antibiotic contamination in food and food products has made it imperative to develop analytical methods for their detection. Polyclonal antibodies and protein-hapten conjugates to cephalexin were used in the development of a surface plasmon resonance (SPR)-based inhibition immunoassay to cephalexin. A conjugate consisting of cephalexin-bovine serum albumin (BSA) was immobilised on the dextran gel surface. Dissociation between the antibody and antigen was easily achieved with 10 mmol l<sup>-1</sup> NaOH and was very reproducible. Standards of free hapten were prepared and premixed with antibody and, after a suitable incubation time, passed over the surface of the chip with the protein-hapten conjugate immobilised. The hapten in solution inhibited the binding of antibody to the surface resulting in higher response units of antibody bound at lower concentrations of free drug. Model inhibition immunoassays to cephalexin were developed in PBS and spiked milk samples. These assays had detection ranges between 4.88 to 2,500 ng ml<sup>-1</sup> and 244 to 3,900 ng ml<sup>-1</sup>, respectively, with reproducible results.