Escherichia coli strains causing diarrhoea in Swedish piglets were isolated; this investigation was made over a 20-year period, from 1964 to 1984. Many of the isolates belonged to O-groups 8, 141 and 149. These strains were separated into different phenotypes with the aid of a new method, "biochemical fingerprinting". This method has been especially designed to sort E. coli-isolates into various phenotypes based on a pattern of quantitatively measured biochemical reactions. It was found that most pathogenic isolates carrying O-antigen 8 belonged to the same phenotype. This phenotype was common in the 1960's, but later it disappeared from the population and was replaced by a wide variety of different phenotypes, most of them non-enterotoxinogenic, but still belonging to O-group 8. In O-group 141 one phenotype dominated in the 1960's, but in the 1970's a new phenotype appeared, which further increased in number in the 1980's. By contrast, in the O-group 149 practically all strains isolated during the 20-year period were found to carry the relevant virulence factors and belong to the same phenotype.