Lymph nodes obtained from 9 patients with active rheumatoid arthritis were stained by immunological techniques permitting identification of B and T cell areas within the nodes. The lymph nodes were also compared with those obtained from patients with lymphoma. The rheumatoid lymph nodes showed prominent B cell staining in the follicular centres by the EAC and fluoresceinated immunoglobulin techniques. Interfollicular areas were intensely stained with anti-theta globulin. In contrast, lymphoma nodes stained less intensely with fluoresceinated immunoglobulins but stained like benign nodes with EAC. The difference in staining may reflect the degree of disorganisation of the neoplastic follicle and/or the different sensitivity of these 2 methods in the detection of B cells. The differences in staining are helpful in distinguishing the benign changes associated with chronic inflammation from neoplastic B cell proliferative changes seen in lymph nodes.