Various aspects of the allergic reactivity of Amerindians in the Venezuelan sector of the Amazon basin were examined. The frequency of positivity in immediate hypersensitivity skin tests with extracts of common local environmental allergens was found to be extremely low (6.7%). As sera from significantly higher proportions of the study group contained specific IgE antibody against the test allergens, and their histamine-induced skin responses were normal, these results support previous suggestions of an inhibited expression of allergic reactivity in such populations. Indeed, the intense helminthic infections detected, and the extremely high total serum IgE levels measured (geometric mean 13,088 IU/ml) indicate the possible occurrence of mast cell saturation by parasite-induced IgE. However, despite a similar lack of agreement between the in vivo and in vitro tests for allergic reactivity against Ascaris lumbricoides in these subjects (43.5 and 97.9% positive, respectively), their extremely high responsiveness to the helminth allergens presents a marked contrast to that against the other environmental materials. Factors other than helminthiasis (e.g. racial, cultural, nutritional) might, therefore, also modulate the expression of allergic reactivity in such populations.