The capsular polysaccharide (CPS) of group B streptococci (GBS) is an important virulence factor that also serves to protect cells from nonspecific host defense mechanisms. Expression of CPS by GBS, as with other encapsulated bacterial pathogens, is not constitutive but varies during growth in vitro and in primary cultures isolated from different sites of infection. Despite this understanding, little is known about regulation of this surface-expressed carbohydrate antigen in GBS. Here we report that expression of type III CPS by GBS strain M781 grown in continuous culture with a modified chemically defined medium is regulated by growth rate. Cells in steady state at mass doubling times (tds) of 0.8, 1.4, and 1.6 h expressed an average of sixfold more cell-associated CPS than did cells held at tds of 2.3 and 11 h. Strain M781 grown at a td of 1.4 h repeatedly produced more type III CPS than those held at a td of 11.0 h, even when limited for glucose, pyridoxamine, or thiamine. In our studies, > or = 93% of the total CPS expressed by strain M781 was cell associated. Strain M781 grown at a td of 11.0 h (i.e., lowered CPS expression) was susceptible to in vitro complement-mediated opsonophagocytosis and killing by human peripheral blood leukocytes, whereas cells grown at a td of 1.4 h (i.e., higher CPS expression) were not killed unless type III CPS-specific antibody was present. Factors that allow GBS to asymptomatically colonize women yet cause invasive infection to both mother and infant are poorly understood. Our results shed new light on parameters that regulate the pathogenic potential of GBS and may also serve as a way to discern more fully the genetics and biochemistry of GBS capsule synthesis.