Groups 3 and 4 will consist of seronegative infants and children. Group 3 will not begin enrollment until the completion of Group 2 safety data review. Children in Group 3 will be randomly assigned to receive the lower dose of rB/HPIV3 or placebo. Group 4 will not begin enrollment until the completion of Group 3 safety data review. Children in Group 4 will be randomly assigned to receive the higher dose of rB/HPIV3 or placebo. Participants of both Groups 3 and 4 will be monitored closely for 28 days postvaccination for respiratory illness and for fever by self-reported temperature logs; these participants will be followed for a maximum of 56 days. Blood collection will occur at study entry and on Day 56. Clinical assessments and nasal washes will occur at study entry and most study visits. Participants' parents or guardians will be contacted by phone periodically post vaccination; study staff will ask about any illnesses or adverse events they may have observed in their infants or children.