Reports of 2 cases of histoplasmosis are presented, together with the clinical findings and a summary of the pathology encountered. One case exhibited tuberculosis as well as histoplasmosis. Both cases were treated with penicillin without evidence of any clinical effect. Raw sputum containing type 6 Pneumococcus and Hemophilus influenzae as well as Histoplasma capsulatum could be treated so as to contain 4 units of penicillin per ml. and be injd. into guinea pigs. Such pigs became infected with H. capsulatum, but were protected from the other 2 organisms which killed unprotected guinea pigs in 24 hrs. In vitro studies demonstrated that H. capsulatum is not inhibited by a conc, of penicillin of 10 U. per ml. of broth culture medium.