Four patients with defective yeast opsonisation and protracted diarrhoea are reported. Plasma infusions improved the opsonising function in all 4 and the diarrhoea in 3. This immunological abnormality was assessed in 100 sequential patients with chronic diarrhoea associated with various gastrointestinal disorders; 52 with protracted diarrhoea and failure to thrive of undetermined cause, 26 with 'toddler diarrhoea', 8 with coeliac disease, 5 with chronic inflammatory bowel disease, and 9 with miscellaneous disorders. 23% of the patients with protracted diarrhoea of undetermined cause had defective opsonisation, a greater proportion (P less than 0.05) than that in 'toddler diarrhoea' or the remaining patients, in whom the frequency (4%) was similar to that (5%) in healthy populations. We suggest that yeast opsonisation be tested in children with protracted diarrhoea, as plasma infusions can be an effective form of treatment.