Infiltration of eosinophils into the airways plays a central role in the pathophysiology of asthma. Human blood eosinophils express apoptosis-inducing receptors (e.g. CD95R and CD69R) regulating both viability and survival and, thus, the extent of eosinophil infiltration into the airways. Signal transduction processes induced by occupation of the CD69 receptor expressed by eosinophils are insufficiently known. Purified human peripheral blood eosinophils (MACS, purity > 99%) were pre-incubated with a GM-CSF for 18 h and stimulated with alpha-CD69mAb (clon TP1/55), alpha-CD95mAb (clon CH-11), and as a control alpha-CD11bmAb (clon Bear-1). The specificity of receptor ligation was assessed using a blocking mAb (Klon ZB4). Phenotype, viability, apoptosis and bcl-2-expression were measured employing flow cytometry. alpha-CD95mAb (1 microgram/ml) induced apoptosis both in control and GM-CSF (10 ng/ml) treated eosinophils. Similarly, alpha-CD69mAb (10 micrograms/ml) induced apoptosis of GM-CSF-stimulated CD69+ cells after an incubation period of 114 h which was not affected by a CD95 blocking mAb. Naive eosinophils showed a basale, bcl-2-expression, which decreased to 30% after 66 h. In the presence of GM-CSF, intracellular bcl-2-concentration remained unchanged. Following stimulation with alpha-CD69mAb or alpha-CD95mAb, a dose-dependent decline of the bcl-2-expression was detected, whereas alpha-CD11bmAb (10 micrograms/ml) had no effect. The data suggest that both CD95R- and CD69R-induced apoptosis of human eosinophils involves a bcl-2-dependent signal transduction mechanism.