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A study was conducted to ascertain how much variation in allergenic potency of ragweed pollen occurs among individual plants and different plant populations in a restricted geographic area. Seeds of the short ragweed (Ambrosia artemisiifolia) were collected from 38 different sites in Champaign County, Ill. Plants from these seeds were grown under similar conditions, and antigen E (AgE) was determined on pollen samples harvested from individual plants. The populations from various sites in Champaign County differed by as much as severalfold with respect to mean AgE values. There were statistically significant differences among population means, although large variations occurred among plants within populations. Further research was conducted to learn whether the level of AgE was determined by genetic factors within the plants. Genetic regulation was indicated by the finding that 3 plants which had unusually low levels of pollen AgE all produced offspring with significantly lower mean AgE levels than those of control plants.