We studied the molecular epidemiology of multiply resistant Haemophilus influenzae type b in four day care centers after each had a case of invasive infection due to organisms resistant to four or more antibiotics. Colonization rates among 174 childhood contacts and 27 employees of the four separate day care centers were 28% in children (range, 13.5%-43.7%) and 22% (range, 18.7%-33.0%) in employees. All strains were type b, were biotype I, were type 12 by isoenzyme analysis, and harbored plasmids whose mass was 45 or 52 megadaltons. Restriction endonuclease analysis indicated that the plasmids were closely related to one another but that there were at least three distinct plasmids. The four index strains had two different outer membrane protein profiles and three distinct plasmid restriction patterns; an additional outer membrane protein profile and plasmid restriction pattern were found in carrier strains.