Presence of cytopathogenic effect (CPE) that could be inhibited by an antitoxin to Clostridium sordelli, known to cross-react with Clostridium difficile toxin, was sought in faecal specimens from 101 infants. Of the children, 45 were healthy, while 56 had been hospitalized because of diarrhoea. CPE was found in 12 of the healthy infants and in 5 of those hospitalized. Faecal specimens of these 5 gave a CPE at titres of 10(3-4), whereas in the 12 healthy infants the titres were 10(1-2). Studies on consecutive samples showed that the CPE could persist for between 7-11 weeks up to 9 months and more. Of the 45 healthy infants, 11 harboured C. difficile compared with 6 of the 56 with diarrhoea. In both groups, 3 CPE-positive infants were culture-negative for C. difficile. Four of those hospitalized had recently been given antibiotics; all were negative in both culture and CPE tests. The present study demonstrates that care should be exercised when interpreting the results of cultures for C. difficile and tests for CPE made on faecal specimens in order to establish a diagnosis of antibiotic-associated enterocolitis in infants and children.