During the storage of nonstabilized, dilute solutions of tuberculin (solutions containing no antiadsorption agent) in glass containers and glass or plastic syringes, great losses of tuberculin occur due to the adsorption of tuberculoprotein to the wall of these containers. Using glass vials filled with tuberculin purified protein derivative-14C at the concentration of 5 tuberculin units per dose, the concentration of the nonstabilized tuberculin solution decreased to 60 per cent of its original concentration one day after filling. During withdrawal in disposable syringes with a capacity of adsorption ranging from 0.19 µg to 0.23 µg purified protein derivative per cm2, the concentration of tuberculin decreased further to approximately 50 per cent when it was immediately injected and to approximately 40 per cent, 30 per cent, 20 per cent, and 10 per cent of the original concentration when it was injected after 20 minutes, 1 hour, 4 hours, and 8 hours, respectively. After 24 hours in the disposable syringe, there was very little tuberculin left in the nonstabilized solution (0.2 tuberculin units per dose). By contrast, a control solution of purified protein derivative-14C containing 0.0005 per cent Tween® 80 (5 ppm) incurred no loss under identical storage conditions. The fluctuating loss of tuberculoprotein in dilute, nonstabilized solutions of tuberculin is a very disturbing factor and renders the tuberculin skin test unreliable and nonreproducible. It is therefore essential that only tuberculin solutions known to be stabilized by Tween 80 or another equally efficient antiadsorption agent be used in tuberculosis control programs and by physicians in private practice.