Twelve strains of Nocardia isolated from sputa, bronchial washings and pleural fluids, were studied. The classification was based on the following characteristics: a) acid-fastness; 2) ability to disolve crystals of tyrosine and xantine; 3) hydrolysis of casein; 4) growth in dilute gelatin medium (0.4%) and 5) galactose acidification. Direct cultivation of the clinical material in Czapek liquid culture medium without carbon source and containing a paraffin rod (pariffin-bait technic), as well as the routin T.B. sputum digestion and concentration with NaOH solution followed by its cultivation in Loewenstein's medium, were employed for the isolation of Nocardia. The sensitivity of the strains to tuberculostatic drugs was investigated using the Canetti's method, as was the ability to growth in Sabouraud and lactrimel culture media supplemented with 100 mug/ml of rifamycin. Finally the pathogenicity test was carried out by intraperitoneal and intramuscular inoculation in guinea-pigs. All the strains were classified as Nocardia asteroides. Seven starins were isolated from patiens with lung conditions of no nocardial etiology. The diagnosis of nocardiosis of the lung was demonstrated in the remaining five patiens by microscopical observation of Nocardia in clinical specimens, by the repeated cultures obtained from these materials and, finally, by the fact that an improvement of the clinical symptomatology was obtained by sulfa administration.