The influence of bacterial plasmids on the pathogenesis of the Enterobacteriaceae has, in several instances, been well-documented. Recently, plasmid DNA has been demonstrated in the genus Streptococcus. It is the purpose of this investigation to determine the possible role of these extrachromosomal elements in the pathogenesis and taxonomy of the oral microflora, with particular emphasis on the streptococci. A procedure for transferring plasmid DNA to Streptococcus sanguis, strains Challis and Wicky, by transformation has been established and is being employed for determining phenotypic properties coded by streptococcal plasmids.