Levels of IgE antibody to fire ant whole body extract (FA-WBE) and fire ant venom (FAV) were evaluated in 60 patients with clinical histories of fire ant sting hypersensitivity and 11 frequently stung individuals without significant clinical reactivity to fire ant. An overlap of levels of IgE antibody to FA-WBE and FAV was seen between groups of patients with increasingly severe clinical reactivity; however, the group mean IgE antibody level of patients without significant clinical reactivity differed markedly from the group means of patients with more severe clinical reactivity (p less than 0.001). In 31 patients a good correlation (r = 0.6935) was found comparing FA-WBE IgE antibody level with FA-WBE skin tests, and in 26 patients a good correlation (r = 0.8564) was obtained comparing FAV IgE antibody levels with FAV skin tests. An excellent Spearman rank correlation coefficient (r = 0.9197) was found on comparison of FAV and FA-WBE IgE antibody levels in 71 individuals. Cross-reactivity of FAV and FA-WBE was demonstrated by the ability of these reagents to competitively inhibit the binding of IgE antibody to solid-phase FA-WBE and FAV in the radioallergosorbent test (RAST). From these results we conclude that FAV and FA-WBE both contain relevant allergens important in fire ant-allergic individuals and that skin tests and RAST are both valid as diagnostic tests for fire ant allergy. Finally, there is a good correlation between RAST and skin tests using FA-WBE or FAV as test reagents, since these reagents are highly cross-reactive as demonstrated by their ability to inhibit each other in the RAST.