From the group B streptococcus (GBS) reference strain 090 la Colindale two subpopulations, which differed markedly regarding their capacities for biosynthesis of type-specific polysaccharide, were obtained by separation on a hypotonic Percoll density gradient. In the original strain and the high and low density variants, there was a negative correlation between buoyant density and bio-synthesis of type-specific polysaccharide as determined by ultrastructure and quantitative assays. The invasiveness of these variants was investigated by infecting rabbits via subcutaneously implanted tissue cages. In the animals infected with highly encapsulated bacteria, heavy bacteremia was detected 8 h post-infection, whereas in the animals which received high density bacteria with small amount of capsule, heavy bacteremia was not detected until after five days. All isolates recovered from the blood or organs of these rabbits were of the capsule rich phenotype, indicating a phenotypic shift in the subpopulation of high density bacteria. An apparently similar phenotypic shift was noted in an isolate from a baby with early onset septicemia. There was a dominance of low density bacteria in the isolate obtained from the baby as compared with the colonizing population of bacteria isolated from the cervix of the mother. From these type III isolates, subpopulations with different density maxima were obtained. A reversed shifting towards dominance of less encapsulated, high density bacteria was observed during in vitro passage of these subpopulations.