Following the intravenous injection of 2 X 10(8) killed Salmonella enteritidis into rats, high levels of specific antibodies were detected in the serum and bile after 4 days. The level in serum persisted while that in bile had virtually disappeared by day 7. On the basis of gel filtration on Ultrogel AcA22 the activity in bile was found to be associated with the IgM fraction. The transient nature of the biliary response was further evidenced by the association of the agglutinating activity with a four-fold increase in the concentration of IgM and the presence of complement-fixing bactericidal antibody. Initial evidence suggests that the source of the biliary IgM is from cells within the liver rather than transport of IgM from blood to bile. It is proposed that immune stimulation by blood-borne microorganisms can lead to a pulse of IgM in the bile, providing rapid augmentation of other local immune processes in the gastrointestinal tract.