A serological test for the assay of Neisseria antibody would be a valuable asset in the study of neisserial infections. Such a test, using bentonite particles sensitized with phenol-extracted and acetone-precipitated antigens from Neisseria gonorrhoeae or Neisseria meningitidis, has been developed. It has proved to be sensitive and specific in titrations against species-specific immune rabbit sera.In a survey of 454 human sera received from the Special Treatment Clinic (Ottawa), 77% of male gonorrhoea patients and 78% of the females had circulating gonococcal antibodies detectable by this test. Ninety-six percent of the control sera were negative. It is suggested that the bentonite technique could be used as an adjunct in the diagnosis of gonorrhoea.