Polysaccharide extracts of culture filtrates of Histoplasma capsulatum and Blastomyces dermatitidis, after preliminary trials in laboratory animals to determine optimal dosage, were used for testing a total of 3808 naval recruits from all parts of the country. Simultaneous tests were also given with standard histoplasmin, H-42, in a 1 to 100 dilution. The results with the H. capsulatum polysaccharide and the histoplasmin were so similar that the two antigens appeared to be quantitatively equal, or very nearly so, in the doses used in the present study population. Almost all of the persons who reacted to one antigen also reacted to the other antigen, and, conversely, most of the persons who did not react to one antigen did not react to the other. The conclusion is drawn that the polysaccharide antigen contains at least some of the same reactive substances as histoplasmin. The frequency of reactions to the B. dermatitidis polysaccharide was very low, about 1%, compared with about 20% to histoplasmin. As there is little reason to believe that the polysaccharide antigen was nonreactive, these results suggest that benign blastomycotic infection[long dash]if it exists at all[long dash]is far less frequent in this country than benign histoplasmal infection.