Tuberculin PPD and PPD-Battey skin tests were simultaneously applied to 3,882 employees of Charity Hospital and 408 medical students at Louisiana State University. The PPD was doubtful (5 to 9 mm induration) in 253 of the total 4,290 persons tested (5.9%). In 86 of these 253 persons, the reaction to PDD-Battey was greater than the reaction to PPD, presumably identifying a subpopulation with a falsely positive PPD and therefore at considerably lower risk of developing future tuberculous disease. Of the 408 medical students (average age 24 years), 80 (19.6%) were classified by the skin tests as having atypical mycobacterial sensitization as compared to six (1.45%) who were classified positive or probably positive to Mycobacterium tuberculosis (P less than .001). In the Southeastern United States, where the incidence of atypical mycobacterial infection is relatively high and occurs at a young age, dual skin testing may have its greatest applicability in identifying tuberculous infection when the PPD falls in the "doubtful" 5 to 9 mm range.