As shown in the preceding paper, propolis or bee-glue is the cause of an increasing number of allergic reactions in persons using it in external preparations and cosmetics. Propolis and its main contact allergen, 1,1-dimethylallyl caffeic acid ester, designated LB-1, show strong sensitizing properties in patients as well as in guinea pig experiments. 9 patients have been patch tested with this compound, 8 of whom reacted strongly. Chemical separation of different propolis samples and poplar bud extracts reveal that LB-1 is always present. Poplar bud secretion is the bee's major source for propolis and hence the origin of LB-1. A warning is indicated, in agreement with several other authors, that propolis should not be used in topical products because of its strong sensitizing properties.