Gold sodium thiomalate (GST) has been used for treatment of patients with bronchial asthma. In this study we investigated the effects of GST on interleukin (IL)-5–mediated eosinophil survival in vitro. Blood cells were obtained from patients with bronchial asthma (n = 18). Eosinophils were purified from the blood samples by a Percoll discontinuous method and negative selection of neutrophils. Eosinophils (10⁶ cells/ml) were incubated for 96 hours in 10% fetal bovine serum–RPMI 1640 (Nissui Pharmaceutical Co. Ltd., Tokyo, Japan) with various concentrations of recombinant human IL-5 (2 × 10⁻⁹ to 2 × 10⁻⁵ mmol/L) and with GST (0.01 to 100 μmol/L) in 96-well, flat-bottomed microtiter plates at 37° C in a humidified 5% carbon dioxide atmosphere. Eosinophil viability was determined by trypan blue exclusion. IL-5 enhanced eosinophil survival in a dose-dependent manner. Higher concentrations of GST inhibited IL-5–mediated eosinophil survival. Moreover, GST induced apoptosis of eosinophils by antagonizing the effects of IL-5. However, GST did not inhibit IL-3– or granulocyte-macrophage colony stimulating factor–mediated eosinophil survival, suggesting that GST selectivity inhibits IL-5–mediated eosinophil survival. These results suggest that IL-5–mediated eosinophil survival is inhibited by GST and that GST blocks eosinophil apoptosis by IL-5. It is possible that GST may be capable of controlling eosinophil functions regulated by IL-5 in patients with bronchial asthma. (J ALLERGY CLIN IMMUNOL 1995;96:251-6.)