Cholesterol can be specifically measured without difficulty and without complex reagents by means of a newly developed enzymatic colour test. Intensive technical evaluation confirmed its accuracy. Manual use gave a day-to-day coefficient of variation of 2-3 per cent; the sensitivity at a cholesterol concentration of 200 mg/dl was E equals 0.153 (gamma equals 405 nm), with a linearity up to 1000 mg/dl. Recovery of added pure cholesterol solution was 100 plus or minus 2 percent. A quantitative study of 53 representative drugs, anti-coagulants and metabolites was performed both in test tube and on patients. Accuracy of the result was unaffected by any of the substances (alpha equals 0.05). Comparison with the multi-step extraction method used at present as a reference (Abell-Kendall) gave a regression equation of y equals 0.99 times plus 1.1 (x axis: extraction method; y axis: enzymatic colour test). The direct chemical method after Liebermann and Burchard gave, in part, markedly differing results because of considerable systematic and accidental errors.