Anti-malarial antibodies were quantified in cerebrospinal fluid (CSF) of 17 cases of cerebral malaria, 16 presumptive cases (no demonstrable parasitaemia in peripheral blood but responding to i.v. quinine therapy) of cerebral malaria, and 15 controls. A schizont-enriched Plasmodium knowlesi antigen was used in an ELISA. Anti-malarial antibodies of IgA and IgM isotypes were not detectable in most of the CSF samples analysed, although serum antibody titres were high. However, 88% of CSF from cerebral malaria and 56% of presumptive cerebral malaria cases had significant levels of IgG anti-malarial antibodies in comparison to control CSF. The antibody levels did not correlate with the severity of coma but correlated well with the duration of coma. The CSF malarial antibody titres were independent of degree of parasitaemia. The possible role of CSF anti-malarial antibodies in cerebral malaria in the light of recent demonstrations of intrathecal synthesis of immunoglobulins and deposition of immune complex in cerebral tissues is discussed.