In a hypogammaglobulinaemic patient with faecal cultures persistently positive for Campylobacter jejuni it was shown that C. jejuni bacteraemia was responsible for a long history of self-limiting attacks of fever. A focus of infection outside the gastro-intestinal tract was not found. Antimicrobial treatment failed to eradicate C. jejuni. Antibodies against C. jejuni were not detectable and there was also a defect in serum bactericidal activity. In contrast with normal serum, it was shown that, when patient's serum was used in tests, IgG and the components of complement C3 and C4 did not bind to C. jejuni.