We analysed the proliferative activity of synovial lining cells (SLCs), the distribution of proliferating B and T lymphocytes and the relationship of proliferating B and T lymphocytes to the pattern of antigen-presenting cells (APCs) within the rheumatoid synovial tissue (n = 21). The immunohistochemical detection of the proliferation-associated antigen Ki67 revealed low proliferative activity of SCL with and without expression of the Kim 8 (CD68) antigen. Ki67-positive B lymphocytes could be observed within secondary follicles (2/21), in small follicular dendritic reticulum cell (FDC)-containing follicle-like aggregates (7/21) and near the enlarged synovial intima (6/21). Ki67-positive T lymphocytes could be detected in T-lymphocyte aggregates (8/21), in the vicinity of blood vessels (18/21) and within the enlarged synovial intima (15/21). Semiquantitative analysis showed a strong correlation between the numbers of Ki67-positive B lymphocytes and FDCs and between the numbers of Ki67-positive T lymphocytes and interdigitating dendritic reticulum cells (IDC). There were significant differences in the number of Ki 67-positive B and T lymphocytes, IDCs and FDCs between the two groups of rheumatoid arthritis (RA) patients with different local clinical activity. These findings demonstrate a low proliferation of SLCs with and without expression of the monocyte-specific antigen Kim 8 and imply that B and T lymphocyte proliferation occurs in the presence of FDCs and IDCs. These results indicate that the RA synovial tissue is a site for antigen-dependent proliferation and maturation of B and T lymphocytes. The atypical pattern of FDC distribution within the rheumatoid synovial tissue "dysmorphic follicle" may be regarded as morphological substrate for a dysmaturation compartment of B lymphocytes leading to pathogenetic autoimmune phenomena in RA patients.