Human eosinophils become hypodense and express class II major histocompatibility (MHC) molecules when activated by granulocyte-macrophage colony-stimulating factor (GM-CSF) in vitro or in vivo in pathological conditions such as allergic disorders. In this study, we examined the capacity of class II MHC-expressing eosinophils to serve as antigen-presenting cells (APC) for resting and activated CD4+ T cells. Eosinophils were isolated from healthy donors and incubated in conditioned medium (CM) containing GM-CSF for 2-4 days, after which 15-92% of the cells expressed class II MHC (HLA-DR). Preincubated eosinophils induced resting T cells to proliferate in response to the staphylococcal superantigens, Staphylococcus enterotoxins A, B and E. Furthermore, superantigen-induced T-cell proliferation correlated with the proportion of eosinophils expressing class II MHC molecules. When eosinophils and macrophages were compared for their ability to act as accessory cells for superantigen-induced T-cell proliferation, macrophages were more efficient than eosinophils. Eosinophils were not effective APC for microbial antigens (Ag), which required processing. Proliferative responses to purified protein derivative, tetanus toxoid, or Brugia malayi antigen were observed in only three of nine studies. The three positive studies included activated CD4+ T cells, whereas no responses were observed with resting CD4+ T cells. Macrophages and mononuclear cells were effective APC for these Ag for both resting and activated CD4+ T cells. These data indicate that although class II MHC-expressing eosinophils can serve as APC, they are relatively inefficient for the activation of CD4+ T cells by Ag, which require processing.