6 healthy adult volunteers were immunized perorally with live Escherichia coli O4K12 bacteria. The bacteria could thereafter be identified in the stools of all individuals for different periods of time. No adverse reactions were noted. 04 antibodies of the IgG class appeared in the serum of all individuals and IgM antibodies in 2 of them. The corresponding figures for K12 antibodies were 2 and 1, respectively. In saliva from the glandula parotis, secretory IgA antibodies to 04 were seen in 3, and against K12 in 1 individual. It is concluded that peroral immunization might be an alternative for induction of antibodies. Nothing is known, however, about any protective effect of these antibodies. Only a low percentage of the individuals showed K12 antibody formation. Therefore, optimal prophylactic immunization against acute pyelonephritis could be achieved by giving immunogenic K antigen, that is conjugated to a carrier, which might induce antibody formation in most individuals.