A synthetic thymus-independent (TI) antigen, dinitrophenyl-'Ficoll', was used to immunise controls, untreated patients with Hodgkin's disease (HD), and patients who had had splenectomy (most for staging in HD). The antibody response to DNP-ficoll was much lower in splenectomised patients than in the other groups and the antibody titres 7, 14, and 21 days after immunisation of splenectomised patients were approximately a tenth of the titres in the other groups. In contrast patients who had been primed with DNP-ficoll before splenectomy responded normally to re-immunisation 6-28 months after splenectomy. These results suggest that the antibody response upon first exposure to certain TI antigens occurs mainly in the spleen. Antibody responses are not generally impaired in splenectomised patients immunised with pneumococcal or other bacterial polysaccharides, probably because natural exposure and priming by these TI antigens before splenectomy are widespread. However, such patients must be vulnerable to overwhelming infection by encapsulated bacteria not met before splenectomy.