Transfer of five plasmids was studied in 30 different mating systems, involving streptococci of groups A, D and H. The erythromycin resistance marker (Emr) of all five plasmids was transferable at frequencies ranging from 8 X 10(-7) to 2 X 10(-4). All original donor strains except SM60 (pERL1) contained phage activity, but evidence was obtained favouring conjugation over transduction as the mechanism of transfer. The process was rec-independent. The transconjugants were able to serve as donors of the Emr-marker in secondary matings. A shortened derivate (pSM10) of the plasmid pSM10419 could also be transferred. Finally, the presence of the plasmid pSM15346 influenced the expression of the capacity to adhere to human epithelial cells and the resistance to UV-light of the host, whereas anti-phagocytic activity and production NADse and of opacity factor was unaffected by the plasmid.