Immunohistological analysis using monoclonal antibodies in conjunction with histochemical techniques has been applied to lung biopsy material from patients with cryptogenic fibrosing alveolitis. Subsets of lymphoid and non-lymphoid cells have been identified in situ. This analysis showed that the inflammatory cells present were predominantly mononuclear. Most of the lymphoid cells were B lymphocytes, organised into follicles with occasional germinal centre formation. IgM was the major class of immunoglobulin expressed. Both T4+ and T8+ lymphocytes were seen diffusely distributed in the interstitium. The T4+ positive cells were also seen within the B lymphoid follicles. Almost all non-lymphoid cells expressed the phenotype of inflammatory macrophages, but a few also expressed a phenotype characteristic of interdigitating cells. These results suggest that a local B lymphoid immune response is occurring in cryptogenic fibrosing alveolitis. The possibility that a cell mediated immune response is also emerging is discussed.