Consecutive serum samples from 61 patients with Clostridium difficile diarrhoea were investigated for antibody response to C. difficile toxins A and B in an indirect enzyme immunoassay (ELISA) and in a neutralization assay against C. difficile cytotoxin. Sera from 64 blood donors, elderly healthy females and patients with other known intestinal enteropathogens served as controls. An immune response was detected by ELISA in approximately half of the patients with C. difficile diarrhoea. The specificity of the ELISA was 94% or 97%, depending on the control material used. Furthermore, a correlation was found between clinical recovery without relapse of C. difficile diarrhoea and high IgG titers to toxin B in the ELISA, and/or appearance of neutralizing antibodies. It is concluded that the ELISA for detection of serum antibodies to C. difficile toxins may be of diagnostic value in combination with the conventional tissue culture assay for cytotoxin in stool. High ELISA IgG titres to toxin B and/or the appearance of neutralizing antibodies may also be a positive prognostic sign in patients with C. difficile diarrhoea.