Abstract: An individual blows into a chamber having a predetermined volume. In the chamber is mounted a gas cell including a heating conductor for catalytic burning of the alcohol and a probe at a predetermined distance from the heating conductor, both embedded in a ceramic porous shell. The lead-in conductor for the probe is connected to the base of a transistor in a threshold circuit which is responsive to the resistance of the gas cell which in turn decreases with increasing alcohol content. When the resistance is less than a predetermined resistance a relay in the transistor circuit operates, lighting an indicator lamp and, in one embodiment, deenergizing the ignition circuit of a vehicle in which the device is mounted.