In a very large collection of recent clinical isolates of GBS that we serotyped, a relatively small percentage, 4.2%, was nontypable. Only 1.4% of 211 GBS early-onset invasive isolates were nontypable, supporting that defined capsular polysaccharide is highly associated with invasive infections. It was quite useful to use the surface localized protein patterns as phenotypic markers to permit us to segregate the strains into groups. Molecular characterization of the isolates and study of their DNA restriction profiles permitted special emphasis on the NT/R1,R4 strains based on their suggestion of clonality by PFGE. These studies supported previously published observations of ours showing remarkable molecular heterogeneity among GBS strains of the same serotype. The PFGE profile of serotype V strains, as well as that of the NT/R1,R4 strains, shared remarkable evidence of clonality and are an exception to the usual findings. Capsule gene probe studies detected the presence of genes described for capsular polysaccharide synthesis in type III GBS, and our results suggested that they are present also in other serotypes, such as type V. Many of these nontypable GBS strains may possess either very small amounts of capsular polysaccharide or they are variants of the classical serotypes, and further genetic, physiologic, and biochemical studies are warranted to better understand the antigenic diversity among GBS strains.