Group B streptococcus (GBS) strains isolated from infected infants are more likely to produce high levels of extracellular hyaluronidase. We hypothesized that administration of exogenous enzyme would promote GBS invasion in newborn rats. GBS strain 1733 which fails to produce the enzyme was grown to early log-phase in COM, washed, and resuspended to 1 × 109cfu/ml in 0.87% saline. Following tracheal exposure, newborn rats were intratracheally injected with 0.5 μl/g of body weight of GBS mixed with 0.5 μl/e of either saline or hyaluronidase solution (42 units/μl). After 2 or 6 h, animals were sacrificed for quantitative bacterial cultures of whole spleen and left kidney. All neonates bad positive cultures. The GBS colonies of spleen and kidney at 6 h were significantly higher than those at 2 h in both groups and the enzyme group had a higher number of GBS than the nonenzyme group at 6 h. The results show that GBS is readily disseminated from the lungs and multiplies rapidly in organs of the neonates following intratracheal inoculation and suggest that hyaluronidase contributes to GBS invasion.