Abstract: A neutral starter interlock for motorcycles, a form of electronic safety circuit, is disclosed. The interlock is designed as a retrofit especially for Harley-Davidson® Big Twin and Sportster® motorcycles manufactured prior to model-year 2007 and factory equipped with an electric starter and a neutral indicator light. The interlock circuit interconnects the neutral indicator circuit (between the battery and the transmission neutral switch) and the starting circuit (between the battery and the starter solenoid) of the motorcycle, and includes an electronic relay that bars current from flowing from the battery to the starter solenoid unless the motorcycle's transmission is in neutral. This prevents unanticipated movement of the motorcycle if an attempt is made to start the engine with the transmission in gear.