1. Serum bactericidal activity against the homologous organism was determined in patients with urinary tract infections and has been related to the site of infection and to serum antibody titres.
2. A significantly higher number of patients with upper compared to lower urinary infections were found to have either a specific defect in bactericidal activity against the homologous organism or to be infected with organisms inherently resistant to normal serum bactericidal activity.
3. It is suggested that these factors may be important in determining the site of urinary tract infections.