The characteristics of cellular elements in the middle ear effusions, peripheral blood, and adenoidal tissue and ther effects of middle ear macrophages on the functional activity of lymphocytes were examined in 50 subjects with chronic otitis media. Macrophages were the predominant cell type in middle ear effusions, and they constituted up to 63% of the total number of cells. T cells were 11%-66% of the total number of cells. The proliferative responses induced by phytohemagglutinin or pokeweed mitogen in middle ear lymphocytes were generally low compared with the responses observed in peripheral blood and adenoidal lymphocytes. Cocultures of middle ear macrophages with adenoidal lymphocytes resulted in a significant depression of proliferative response and immunoglobulin synthesis in the lymphocytes. These observations suggest that macrophages in the middle ear may have a profound influence on the regulation of the immune response in the middle ear in patients who have otitis media with effusion.