This study will assess the immunogenicity and safety of the GlaxoSmithKline (GSK) Biologicals' (formerly SmithKline Beecham Biologicals') combined DTPa-HBV-IPV/Hib (Infanrix hexa™) vaccine administered in the 3rd, 5th, 11th month of life. The last dose of DTPa-HBV-IPV/Hib will be given simultaneously with one dose of OPV vaccine.