DURING the last 40 years less than 100 proved cases of histoplasmosis have been reported in the world literature. These few cases, nearly all fatal, cannot be considered a significant public health problem. Public health interest is aroused, however, by Christie and Peterson1 and Palmer,2 3 who advance the hypothesis that human histoplasmosis may exist in a mild form and may be an important factor in the etiology of calcified pulmonary lesions. The general background of this hypothesis lies in the reports of Gass, et al.,4 Lumsden and Dearing,5 Olson, et al.,6 and others, showing that a large number of persons have pulmonary clacification not associated with demonstrable tuberculosis. Olson, et al.6 concluded that such calcification must be due to some unrecognized disease of common occurrence other than tuberculosis. Thus, when Christie and Peterson, Palmer,2'8 and Furculow, et al.7 found that a large proportion of the population in certain areas gave positive skin reactions to a filtrate of Histoplasma capsulatum grown on a synthetic medium, namely histoplasmin,t and also