Comstock, G. W. (Training Center for Public Health Research, Box 2067, Hagerstown, Md. 21740) C. N. Vicens, N. L. Goodman, and S. Coilins. Differences in the distribution of sensitivity to histoplasmin and isolations of Histoplasma capsulatum. Amer. J. Epid., 1968, 88: 195–209.—In 1963, 7,787 junior and senior high school students in Washington County, Maryland were tested with histoplasmin. Among 1,450 students who were lifetime residents of a single house, there was marked geographic variation in the prevalence of histoplasmin reactors, ranging from 10 to 90% with no apparent correlation with any topographic or geographic features except for low prevalence within the city of Hagerstown. Marked variation in the frequency of histoplasmin sensitivity within short distances made it unlikely that meterologic factors were involved and suggested that infection was usually acquired close to home. No association was found with the presence of chickens or domestic birds on the premises. Students who lived on alkaline soils were more likely to have been infected than those who lived on acid soils. Positive soil cultures for Histoplasma capsulatum did not show a similar pattern of associations, being more frequent from premises with chickens and in areas with acid soils. The discrepancies between skin sensitivity and soil isolations could result from some characteristic of the organism, such as spore size, which varied with soil conditions.