During a two-year period data were collected nationwide in The Netherlands on 438 episodes of bacterial endocarditis (BE) in 432 patients. Of the strains isolated in these patients 419 were available for analysis. Of these, 326 were isolated in native valve endocarditis (NVE) and 93 in prosthetic valve endocarditis (PVE). Viridans streptococci, staphylococci and enterococci together constituted 87% of the isolates. More than 46% of the viridans streptococci consisted of Streptococcus sanguis. Enterococcus faecalis and Staphylococcus aureus were the predominant species in the late form of PVE. The majority of the viridans streptococci and haemolytic streptococci were highly susceptible to penicillin. Five of 35 strains of coagulase negative staphylococci were resistant to methicillin. Eleven percent of a random sample of the streptococci collected were tolerant to penicillin. After repeated exposure to a concentration gradient of an appropriate beta-lactam antibiotic, this figure increased to 49%. Of the staphylococci, 5-6% of the strains were tolerant before induction and 16-20% after induction. Of the Enterococcus strains (n = 40), 12.5% showed high-level resistance to one or more aminoglycoside.