A prospective assessment of the bacterial complications of endoscopic retrograde cholangiopancreatography (ERCP) was made in 97 unselected patients undergoing 101 endoscopies. Blood cultures were taken before and five and ten minutes after completion of the procedure. In an attempt to identify a potential source of infection, aspirates were taken from the pancreaticobiliary duct (PBD) after injection of contrast material. Blood cultures from all procedures were negative. In 14 patients PBD aspirates yielded Pseudomonas aeruginosa, pyocin type 22 and serotype O6, suggesting a common source for this organism. Isolation of this strain ceased after more rigorous cleansing and disinfection of the endoscope. The occurrence of bacteremia following ERCP is low, but there is a risk of transmission of potential nosocomial pathogens with this procedure.