With a view to further improving the tuberculin patch test, purified protein derivative was substituted for old tuberculin. Purified protein derivative was supplied in a 1 per cent solution by the Henry Phipps Institute, Philadelphia, with the information that this solution has the same strength as a standard preparation of old tuberculin. According to several authors, this product is superior to old tuberculin.
Filter papers were saturated with old tuberculin from the New York State Department of Health and with purified protein derivative, respectively; they were then dried and cut into squares measuring 1 by 1 cm. One of each kind of these squares was pasted on every adhesive strip. In this way ideal conditions for a comparative study were obtained, since the two products were applied in the same concentration, at the same time, on the same area of skin of the same subject. These combined patch tests