Urinary levels of morphine and codeine were studied in two groups of people: Group A--urine samples of 76 subjects receiving three different medicinal preparations containing morphine or codeine. Group B--urine samples of 67 drug abusers detected at urinary mass screening. Distinct differences in these levels were observed. In Group A, UPPER 99% Confidence limits of morphine concentrations of 2.56 microgram/ml, 2.40 microgram/ml and 2.29 microgram/ml were reached after consumption of prescribed doses of Syrup Phensedyl, Tablet Codeine Co and Kaolin et Morphine mixture respectively. In Group B, the LOWER 99% Confidence limit of morphine concentration was 3 microgram/ml. In contrast, the codeine levels obtained for both groups were similar. The clear separation of the ranges of morphine values for the two groups shows that the urinary morphine level forms a reliable index for the differentiation between these two groups.