Two experimental mouse models were tested for their suitability in measuring virulence of two human and two bovine isolates (types Ia and III) of group B streptococci. In the first model, the kinetics of the number of bacteria in the spleen, liver, and placenta of mice inoculated intravenously on day 16 of pregnancy were monitored for 48 h after infection. In the second model, lethality and abortion were recorded for mice inoculated on day 13 of pregnancy. Levels of colonization in spleens or livers and lethality were significantly greater (P less than 0.001) for human isolates than for bovine isolates. In contrast, no statistically significant differences in the ability to colonize placentas and to induce abortions were noted between human and bovine isolates. The results showed that pregnant mice were more sensitive than nonpregnant mice to a challenge with group B streptococci. The results also suggest that placental colonization and abortion could be a suitable mouse model in evaluating the virulence of human and bovine isolates of group B streptococci.