Eosinophil infiltration of the airways in response to allergen exposure is a characteristic of bronchial asthma. However, the mechanisms by which these cells are recruited are poorly understood. We have investigated the presence of eosinophil chemotactic activity (ECA) in bronchoalveolar lavage fluid obtained from allergic asthmatics (n = 6) 4 h after endobronchial allergen challenge. ECA was purified by sequential heparin affinity chromatography and reverse-phase HPLC. A single peak of ECA was detected; SDS-PAGE and Western blot analysis showed that the peak contained a protein of 8 kDa and corresponded to the chemokine RANTES (regulated upon activation, normal T cell expressed and secreted). Consistent with this, the ECA was neutralized by an Ab to RANTES. Measurement of RANTES by ELISA in 10x concentrated bronchoalveolar lavage fluid showed increased levels of this chemokine at the allergen site (median, 187 pg/ml; range, 46-263 pg/ml) in comparison with a saline challenge control site (median, 32.5 pg/ml; range, 11-94 pg/ml), P < 0.005. Furthermore, there was a significant correlation between concentrations of immunoreactive RANTES and the number of eosinophils at the allergen challenge site (r = 0.8; p < 0.001), but not at the saline site (r = 0.2; p = 0.12). These results suggest that RANTES in involved in the recruitment of eosinophils into the asthmatic airways after allergen challenge.