Restriction fragment length polymorphism of the gene encoding rRNA (ribotyping) was used in combination with conventional epidemiological markers to study phenotypic variations among Streptococcus agalactiae of bovine origin and the possible epidemiological interrelationship between the bovine and human reservoirs of Streptococcus agalactiae. The bovine material constituted 53 strains (9 antigen combinations) isolated from 11 herds. Herds with a uniform as well as heterogenic antigenic pattern were included. Furthermore, strains isolated in the course of time from the same persistently infected quarters were examined. The human material constituted 16 strains, 4 each of 4 serotypes, isolated from healthy carriers. Finally, nine serotype- and the group reference strains were examined. All strains were serotyped by double diffusion in agarose gel, biotyped (lactose +/-), and ribotyped using two restriction enzymes, Hind III and HhaI. All isolates could be typed by ribotyping and seven ribotypes were identified among the reference strains. The restriction enzymes used alone or in combination gave typing results that allowed discrimination between and within serotype. Combined use of serotype, Hind III and HhaI ribotypes produced 11 types among the 16 human strains. Ribotype analysis discriminated between herds infected with the same serotype. Strains of varying antigenic patterns from the same herd had the same ribotype. Phenotypic variations in serotype observed in persistent intramammary infection were not related to genetic changes as monitored by ribotype. Two ribotypes were represented among both bovine and human strains. The discriminating capability of lactose fermentation was of limited value.