Streptococcus adjacens and S. defectivus are two recently individualized species of viridans streptococci. Both species were previously designated as nutritionally variant streptococci and characterized by their growth as small satellite colonies supported by 'helper' bacteria, or requiring complex media enriched with cysteine or with pyridoxal. Structural abnormalities were observed under these conditions and balanced growth can be obtained only with a semi-chemically defined medium, in which the cells are morphologically normal and exhibit a parietal chromophore. The two species are primarily isolated from the oral cavity, or from intestinal and genito-urinary tracts. They are likely to be responsible for most blood-culture-negative endocarditis and account for more than 4% of streptococcal strains isolated from blood cultures during this disease. Cases of endocarditis caused by these bacteria exhibit much higher rates of failure, relapse and mortality than those due to other viridans streptococci. The slow growth rate of the bacteria and the production of large amounts of exopolysaccharide in vivo may account for the difficulties encountered in treatment and suggest that a longer course of antimicrobial therapy may be required for successful cures. Results obtained from experimental models of endocarditis confirmed the efficacy of vancomycin as an alternative therapy in patients when a combination of penicillin and aminoglycoside is ineffective or contraindicated.