A new sensitive gas sensor based on a self-assembled monolayer field-effect transistor (SAMFET) was used to detect the biomarker nitric oxide. A SAMFET based sensor is highly sensitive because the analyte and the active channel are separated by only one monolayer. SAMFETs were functionalised for direct NO detection using iron porphyrin as a specific receptor. Upon exposure to NO a threshold voltage shift towards positive gate biases was observed. The sensor response was examined as a function of NO concentration. High sensitivity has been demonstrated by detection of ppb concentrations of NO. Preliminary measurements have been performed to determine the selectivity.