Peripheral blood mononuclear cells of cystic fibrosis patients without a history of diarrhoea [n=4; median age 23 yrs (20-26 yrs)] and non-CF patients with C. difficile-associated diarrhoea [n=20; median age 67 yrs (32-96 yrs), blood samples obtained within 10 days of the onset of diarrhoea] were exposed to fluorescently-labelled toxin A488 and labelled with antibodies to CD19 (pan B cell marker) and IgD (expression lost in switched memory B cells). This evidence concerns the gene CD19 and cystic fibrosis.