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Tuberculin purified protein derivative labeled with14 C ([14 C]PPD) with a biological potency equivalent to the International Standard for tuberculin PPD was used to study the retention of tuberculin PPD in the skin of sensitized and nonsensitized animals. We found that [14 C]PPD was almost entirely cleared from the skin test site during the first 18 to 24 h after injection and that when approximately 5% of the initial concentration of [14 C]PPD was present in the skin test site, the size of the tuberculin skin reaction in sensitized guinea pigs was at its maximum. Furthermore, the addition of 5 or 50 μg of Tween 80 per ml to a solution of PPD did not change either the rate of clearance of PPD from the skin test sites of sensitized guinea pigs or the size of the tuberculin skin reactions. There was no difference in the rate of clearance of [14 C]PPD from the skin test sites between sensitized and nonsensitized guinea pigs and between guinea pigs of different age. However, there was a significant difference in the rate of clearance of [14 C]PPD between the guinea pig and the mouse. Finally, the percentage of [14 C]PPD retained in the site of injection at 24 h was in the neighborhood of 5% of the initial concentration of the solution of PPD injected. The significance of these phenomena is discussed.

Retention of 14C-Labeled Tuberculin Purified Protein Derivative in the Skin of Sensitized and Nonsensitized Animals