Clinical isolates of Streptococcus faecalis were examined for plasmid content and plasmid-related traits in order to determine whether such strains carried conjugative antibiotic resistance plasmids whose transfer was stimulated by sex pheromones. A 35-Mdalton tetracycline-resistance plasmid called pCF-10 was identified. pCF-10 carries genes that enable its host cell to clump and to act as a highly effective donor when exposed to sex pheromone preparations from recipients. The genetic background of the host cell markedly affects the expression of the pCF-10 fertility functions. The properties of this plasmid make it a good candidate for a detailed genetic analysis of streptococcal conjugation.