1.Repeated mouse transfer did not enhance the virulence for mice of two strains of Blastomyces dermatitidis which had caused human blastomycosis. Virulence was judged by the duration of survival of the animals following intraperitoneal inoculation and the extent of the lesions produced.
2.The yeast form of the two strains was obtained from the mycelial form as readily by growing the organism directly on blood agar at 37°C. as by first passing it through a mouse. The cultural and microscopic appearances of each organism on blood agar at 37°C. were not altered by repeated mouse transfer.
3.The experimental results suggest that the virulence of this fungus, at least for mice, is an attribute which is not readily changed.