Hemophilus influenzae type b is the most common cause of bacterial meningitis in the U.S.; about 13,000 infants and children contract this disease annually. The population at greatest risk consists of children under 2 years of age, but currently licensed vaccines are not recommended for use before age 18 months. The object of this proposal is to evaluate the safety, immunogenicity, and efficacy of a new vaccine developed by NICHD scientists. We plan to obtain information on the preventive effect of a three-dose course of immunization with a new Hemophilus influenzae type b (Hib) vaccine (Hib capsular polysaccharide conjugated with tetanus toxoid) on the incidence of Hib invasive disease in children age 2 months at enrollment, and followed for 2-4 years. We propose to conduct a double-masked, randomized, placebo-controlled efficacy trial, with careful surveillance for symptoms of disease, and adverse reactions from the vaccine.