An outbreak of infection due to Streptococcus zooepidermicus is reported. The index case was a patient who suffered an episode of septicaemia complicated by glomerulonephritis. Two other persons in his family were found to be asymptomatic throat carriers. The source of the outbreak was unpasteurised milk from a house cow on the farm where the family lived. Molecular studies confirmed that the strains isolated from the index case, the other family members and the cow's milk were identical. Although a renal biopsy was not performed, the diagnostic criteria for poststreptococcal glomerulonephritis were satisfied. The organism is an uncommon human pathogen that sometimes causes outbreaks of severe infection which may be associated with glomerulonephritis.