In the laboratory diagnosis of human histoplasmosis and in the research conducted with animals experimentally infected with Histoplasma capsulatum, a variety of methods have been used for the isolation of this fungus. Thus in the laboratory diagnosis of human infections, Pinkerton (1949) in his review recommends direct microscopic examination of stained preparations of biopsied material such as enlarged superficial lymph nodes, cutaneous, muco-cutaneous or naso-oral lesions, aspiration biopsy of the liver, bone marrow, and blood smears. He also suggests that such biopsied material, as well as stools and sputum, should be cultured. The media that Pinkerton recommends are Sabouraud's agar and blood agar, but he also states that the organism grows readily on many artificial media, including Sabouraud's maltose agar, dextrose blood agar, and potato-dextrose medium. The yeast-like phase of the fungus will grow at 37 C in cultures with a high protein content, such as blood or serum agar. The mycelial phase of the fungus will occur when the cultures are incubated at room temperature. The structures that characterize H. capsulatum, namely, the large, tuberculate chlamydospores, occur only in the mycelial phase. In spite of a considerable number of clinical reports and experimental investigations, relatively little has been done to evaluate the various technics and media used in the diagnosis of histoplasmosis and the isolation of H. capsulatum from infected tissues. Howell (1948) has reported on the efficiency of two media (brain-heart infusion blood agar and potato-dextrose agar) when using the spleens of infected guinea pigs. He did not, however, correlate his cultural findings with direct microscopic observation of the fungus in the tissues. This investigation was conducted with the thought of obtaining data on the following points relative to the isolation of H. capsulatum from infected tissues: (1) To determine which tissue (liver, spleen, and heart blood) was most frequently infected with the fungus, both by direct microscopic examination and by cultural examination. (2) To compare the relative efficiency of direct microscopic examination with that of cultural examination. (3) To determine which one of five media (modified Sabouraud's dextrose agar, brain-heart infusion blood agar, brain-heart infusion agar, mycophil agar, and potato-dextrose agar) was most satisfactory for the isolation of H. capsulatum from infected tissue.