The sensitivity and specificity of dual mycobacterial skin testing were assessed in an unblinded study of 22 patients with culture-confirmed Mycobacterium avium complex (MAC) infection and 20 patients with culture-confirmed Mycobacterium tuberculosis infection. Intradermal skin tests were performed with 0.1 mL of M. avium sensitin, 0.1 mL of PPD (purified protein derivative), and two control antigens (mumps and Candida). All patients with M. tuberculosis infection reacted to the skin tests; the mean reaction size was 19.7 +/- 1.4 mm when PPD was administered and 10.3 +/- 1.5 mm when M. avium sensitin was administered. Four patients with MAC were anergic; for the remaining 18, mean reactions of 15.2 +/- 1.4 mm to M. avium sensitin and 4.3 +/- 1.3 to PPD were noted. A skin test was defined as M. avium-dominant or PPD (M. tuberculosis)-dominant if there was a minimum reaction size of > or = 5 mm to the given species, and the reaction to the given species was > or = 3 mm greater than the reaction to the heterologous species. Dominant skin test reactions were present in 18 (90%) of 20 patients with M. tuberculosis and 15 (83%) of 18 nonanergic patients with MAC. The specificity of dominant skin tests was 100% for infection with M. tuberculosis and 100% for infection with MAC. M. avium-dominant skin tests identify subjects with prior MAC infection and distinguish them from patients with M. tuberculosis infection.