A model using human umbilical vein endothelial cells was developed to study the adhesion of group B streptococci. Ten clinical isolates of serotypes Ia, Ib and III, including six blood isolates from neonates with early-onset disease, were studied. Isogenic variants with different ability to express capsule substance were also included. Clinical isolates of serotype III adhered significantly better than other serotypes. Strains with high ability to adhere to endothelial cells also had high ability to bind to plastic. Isogenic variants of serotype III with low amounts of capsule substance adhered significantly better to cells than variants expressing high amounts of capsule substance. These results show that the higher adherence of serotype III strains may be a virulence factor and contribute to the finding that this serotype dominates in invasive group B streptococcal disease.