A prospective study of 806 healthy volunteers was carried out to determine the value of induration 24 hours after placement of purified protein derivative, if patients can accurately read their own tests, and if the pen and palpation methods are comparable. Subjects were actively employed health workers between 20 and 64 years of age without active tuberculosis. All subjects were tested using 5 tuberculin units of purified protein derivative via the Mantoux technique. Induration was quantitated by trained readers at 24 hours and at 48 to 72 hours. Patients were asked to read their own tests as well at 48 to 72 hours. The amount of induration at 24 hours was highly predictive of eventual findings at 48 to 72 hours. The patients' abilities to read their own tests were poor. Significant differences were noted between the pen and palpation methods. The degree of induration noted 24 hours after placement of 5 tuberculin units of purified protein derivative is valuable in a screening population. Most decisions, however, should still be based on results measured at 48 to 72 hours. Results with the pen technique must be interpreted with caution in the 5- to 14-mm range. The patients' readings of their own results are inaccurate and should never be relied on to make clinical decisions.