A new colorimetric procedure for blood alcohol determination is described. Blood proteins are precipitated, and the protein-free supernatant incubated with nicotinamide-adenine dinucleotide (NAD), alcohol dehydrogenase (ADH), nitro blue tetrazolium (nitro BT) and phenazine methosulphate (PMS). Ethanol in the sample reduces the NAD, and the reduced NAD so formed reduces nitro BT to a coloured formazan, PMS serving as an intermediate electron carrier. The reaction is allowed to proceed to completion and colour formation quantitatively relates to sample alcohol concentration. An ethanol standard of known concentration is included with each batch of determinations and sample alcohol concentration calculated from this. The method permits the examination of large numbers of samples with rapidity and precision and without the need for specialized apparatus. Comparison with an ultra-violet enzymatic procedure for blood alcohol determination, gave excellent agreement.