The detection of low molecular weight compounds, such as hormones or antibiotics, is essential for medical treatment and food industry. Commonly used procedures for detecting antibiotics, e.g., penicillin G, often include the use of markers implying extensive sample preparation and time-consuming incubation. particularly the latter is also an issue in microbiological tests. Biosensors, on the other hand, offer a low-cost and rapid alternative detection method. However, in case of penicillin G detection, only a few label-free biosensors have been developed up to now for this purpose. Surface acoustic wave (SAW) biosensors enable the detection of DNA, bacteria and clinically relevant proteins. We will show that they can also be used to detect low molecular weight analytes, such as penicillin G, in short time and label-free. We developed a binding inhibition assay to detect penicillin G (Mr 372). The SAW biosensor surface was modified with penicillin G used as ligand, i.e., antibodyspecific binding partner. Unspecific binding was prevented by coupling the ligands via the hydrogel layer aminodextran. Each penicillin G sample was pre-incubated with the same concentration of antibody, anti-pencillin, for 30 min. Binding of penicillin G (analyte) to anti-penicillin binding sites inhibits the occupied antibody from binding to the ligand on the biosensor surface. Therefore, with increasing concentrations of penicillin G in the sample, smaller sensor signal responses are obtained and vice versa. The surface modification and assay procedures were optimized, aiming at low concentrations to be detected. The resulting assay enabled now the fast and label-free detection of 2-4 ng/ml penicillin G in buffer and low fat milk. This is the maximum residue limit (MRL) for penicillin G in food, e.g., milk, given by the European commission (order 2377/90, 1990).