Human eosinophils are known to lose Ia antigen expression as they mature, and, accordingly, eosinophils obtained from the blood of five eosinophilic donors and three of four normal donors failed to display the major histocompatibility complex class II antigen HLA-DR, as determined by flow cytometry. However, when eosinophils from these nine donors were maintained in culture with recombinant human granulocyte-macrophage colony-stimulating factor and murine 3T3 fibroblasts, HLA-DR consistently developed on the eosinophils. By days 4-6 of culture, 24-97% of eosinophils were HLA-DR+, and the eosinophils remained morphologically mature. In contrast, another class II antigen, HLA-DQ, was not detectable by flow cytometry on eosinophils from eight of nine donors. Cultured eosinophils were able to synthesize HLA-DR, as documented by the incorporation of [35S]methionine into immunoprecipitable HLA-DR heavy and light chains. These findings show that mature eosinophils can synthesize and express HLA-DR and provide a means whereby eosinophils may interact with CD4+ lymphocytes.