The Harderian gland of chickens is known to become infiltrated by plasmacytoid cells 2–4 weeks after hatching. Most of these cells carry immunoglobulin surface determinants (ISD). By indirect immunofluorescence procedures and using heavy-chain-specific antisera to chicken IgM, IgG and IgA, it was possible to delineate the percentage of cells with surface determinants of different immunoglobulin classes. It was found that up to the fourth week after hatching, most of the cells bear IgM as ISD. From the fourth to the ninth week, both IgG- and IgA-positive cells form the bulk of the lymphoid cell population, whereas later on, IgA alone was the predominant specificity of ISD. Cytoplasmatic staining of fixed cells revealed a similar pattern of immunoglobulin class distribution, the IgA-producing cells forming clusters within the stroma of the Harderian gland along and around the excretory ducts. IgG-producing cells were also encountered in groups in the interepithelial regions, but IgM-positive cells occurred singly. IgA was also detected in the cytoplasm of epithelial cells, mostly in the supranuclear portion.