Serum Haemophilus influenzae Type b (HITB) anticapsular antibodies were induced in adult volunteers by feeding of either of two strains of a cross-reacting Escherichia coli 075:K100:H5. In all the fed volunteers colonization for a finite period lasting up to eight weeks occurred, without adventitious reactions. Nine of 14 normal volunteers and one of two volunteers previously immunized with the Type 6 H.influenzae polysaccharide responded with a greater than twofold increase in serum Type 6H. influenzae antibodies. These antibodies induced by Esch. coli were specific for the capsular polysaccharide and had bactericidal activity. The safety of this procedure, the comparable results in laboratory animals and the identification of other cross-reacting strains of Esch. coli with meningococcal and pneumococcal capsular polysaccharides suggest that colonization with these nonpathogenic organisms at birth may provide a general method of preventive immunization to diseases caused by encapsulated bacteria.