Abstract: In a portable breath alcohol detector using a standard tubular mouthpiece, a breath direction and continuity sensor takes the form of a whistle mounted at a predetermined distance from the discharge end of the mouthpiece to leave a gap, open to the ambient atmosphere, between the discharge end and the whistle, the whistle being of sufficient sensitivity to be activated by a breath blown toward the whistle across the gap and the gap being of a size not appreciably to restrict the flow of breath, and permit air being inhaled to pass freely, whereby if the subject stops breathing, or inhales, the whistle stops sounding, and whereby residual alcohol in the whistle will not be drawn back into the mouthpiece when the subject inhales.