Abstract
Summary An indirect fluorescent antibody staining technique was adapted to study the population of potentially pathogenic E. coli serotypes relative to the total coliform population in faeces from calves. Preliminary experiments were carried out with pig faeces in which the haemolytic property of a pathogenic serotype was utilised to facilitate comparison of the results with those obtained from plate counts. In a natural outbreak of colibacillosis in calves, diarrhoea in all except one case was associated with an increase in the proportion of pathogenic serotypes to at least 50 per cent. of the total coliform population. In some other cases, a similar increase occurred in the absence of clinical disease. The significance of these results is discussed in relation to the practical value of the method as a diagnostic procedure and as a means of studying the influence of environmental predisposing factors on the E. coli population of the gastro-intestinal tract.

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