1. Humoral immune capacity was measured in New Guineans with tropical splenomegaly or who had been splenectomized for this disease, to assess relationships between tropical splenomegaly and immunological function; the test antigens were flagellin and monovalent influenza vaccine.
2. Controls included age- and sex-matched healthy and hospitalized Caucasians, and comparative data on responses to flagellin were available for New Guinean school children.
3. For flagellin, geometric mean titres in the primary immune response in New Guineans with tropical splenomegaly were significantly lower than those of other groups, whereas pre-immunization titres and those obtained during the secondary response were comparable.
4. For influenza vaccine, both pre-inoculation and peak mean post-inoculation titres of antibody were significantly lower in New Guineans, attributable possibly to lack of previous exposure to this antigen.
5. The low primary antibody responses in New Guineans with tropical splenomegaly could result from pre-emption of antibody-producing tissues arising from abnormal demands for antimalarial antibody; this may determine the increased susceptibility to, and high mortality from, bacterial infection in tropical splenomegaly.