The combination of an aminoglycoside with a cell wall active agent is often necessary for the satisfactory management of serious enterococcal and streptococcal infections. High level gentamicin resistance (MIC > 1000 micrograms/ml) eradicates the synergy between these two classes of antibiotics and treatment of these infections becomes difficult. Over a four year period we assessed the susceptibility of blood culture isolates of enterococci, Streptococcus agalactiae (group B), and clinically significant viridans-type streptococci. The susceptibility of the organisms to gentamicin could be determined utilizing high content aminoglycoside discs. We report an increasing prevalence of high level gentamicin resistance in enterococci in South London, with 44% of isolates being resistant in the first quarter of 1991. None of the streptococcal isolates demonstrated high level gentamicin resistance. We recommend screening for high level gentamicin resistance in all enterococcal and streptococcal cases where aminoglycoside-penicillin synergy is desired.