A method and an ignition interlock for preventing operation of equipment when an operator's blood-alcohol content is above a threshold value. The interlock has a blood-alcohol detector that measures intensities of wavelengths of light emerging from a finger. A microprocessor correlates these intensities with the finger's blood-alcohol content, determines whether this content is above a threshold level, and prevents the equipment from operating unless the blood-alcohol content is below the threshold. The interlock also has a fingerprint image generator which reflects light of the finger and scans the fingerprint to form a scanned image. The microprocessor compares this scanned image to a prestored image of a principal operator and compares the two images to determine whether the images match. The fingerprint and blood-alcohol analyses occur substantially simultaneously.