A total of 93 lymph nodes from 24 patients with a primary malignant non-Hodgkin lymphoma of B-cell lineage was assessed histologically with special reference to the presence or absence of neoplastic B-cells in the lymph node cortex and medulla (B-cell areas) as well as in the paracortical (T-cell area) area, and in the lumen of the post-capillary venules (PVC). The B-cell origin of the lymphomas was verified by using an indirect immunoperoxidase technique for the demonstration of the cell surface immunoglobulins. The B-cell areas of the node were involved in all the lymph nodes studied, and the T-cell areas in 95.7 per cent of the nodes. Neoplastic cells were present inside the PCV in 80.6 per cent of the nodes. Furthermore, 15 lymph nodes were seen where the paracortical area was only partially involved despite the total replacement of the B-cell areas by the neoplastic cells. The results suggest that the nodal involvement by the B-cell lymphomas starts from the B-cell areas, a fact that could be used as an aid in the diagnosis of these lymphomas as well as in the study of the dynamics of B-lymphocytes in general.