A nocardioform bacterium was isolated from the spleen tissue of an armadillo infected with M. leprae and easily propagated in pure culture in mineral salt medium supplemented with only simple C and N sources (e.g., liquid paraffin, tetradecane, ammonium salts, urea, asparagine, gelatin, xanthin, hypoxanthin etc.). Complex organic substances, e.g., tyrosin, casein, peptone, meat extract, egg proteins, serum, blood, yeast extract as well as medium 199, did not support the growth of this organism. Microscopically, the organism consisted of acid-fast, long, slender rods which originated from long, fragmented hyphae, or sporulating mycelial tufts; it was acid-fast (at less than 4.0% H2SO4) which was pyridine-susceptible. It produced DOPA-oxidase and Catalase and was lysozyme resistant; this grew best under reduced O2 tension, at pH 7.0 to 8.0 and 28 degrees C. Serologically, it appeared to be only weakly related to the prototype human multibacillary leprosy-derived (reference) nocardioform strain, Nocardia brasiliensis and N. caviae, but was variably related to several mycobacteria strains.