Group B streptococcus (GBS), a bacterium with polysaccharide capsule, is the major cause of sepsis and meningitis in early infancy. Recent work has indicated that immunity to GBS infection can be elicited by the surface proteins Rib and alpha, either of which is expressed by most GBS strains causing invasive infections. Here we show that a bivalent vaccine, composed of purified Rib and alpha mixed with aluminum hydroxide (alum), an adjuvant accepted for human use, elicits an antibody response to each of the two antigens. Moreover, the bivalent vaccine was found to protect against experimental infection with GBS strains representing the four classical serotypes. Our results represent an encouraging step towards the development of a human GBS vaccine based on pure protein antigens.