The present report deals with primary antibody responses to tetanus toxoid in 50-54-week-old ('ageing') as compared to 8-9-week-old ('young adult') mice. Antitoxin in the serum appeared 6 days earlier in the older than in the young animals, but in the latter reached 5 times higher titres on day 20. The magnitude of the proliferative response in the paracortex and the medulla of popliteal lymph nodes, as estimated by combined 3H-thymidine autoradiography and planimetry, was 3-7 times greater in the younger than in the older age group, thus approximately reflecting the difference in antibody titres on day 20. In contrast, germinal centre formation in response to the stimulus proved to be about 14 times less in ageing than in young adult mice. The findings demonstrate that, in the model system used, the age-related slopes of decline in humoral antibody responsiveness and proliferative reactivity in paracortex and medulla of first regional lymph nodes tend to be in parallel, while the ability of the immune apparatus to form germinal centres at this site deteriorates at a considerably faster pace. Results are also in line with the notion that centroblasts/centrocytes contribute little, if anything, to the ongoing antibody production elicited by the same stimulus which had triggered germinal centre formation. Finally, the observations made disprove the general validity of the suggestion that immune reactivity is maintained on the same level throughout life if tested with a novel antigen.