A total of 5116 non-fastidious bacterial strains isolated from either blood or urine culture were collected from 20 geographically distributed centres within the UK. Upon receipt the strains were identified and the minimum inhibitory concentrations of ten antimicrobial agents in hospital usage were determined for each of the strains. All laboratories submitted a similar range of bacterial species and in species normally considered as sensitive the overall rates of resistance to the different antibiotics tested were: amikacin 2.4%, gentamicin 3.7%, netilmicin 2.6%, tobramycin 3.4%, ampicillin 41.7%, cefadroxil 11.6%, cefotaxime 1.4% [corrected], cefuroxime 4.3%, ciprofloxacin 0.6% and trimethoprim 13.0%. For those strains resistant to one or more aminoglycosides the mechanisms of resistance responsible were determined from the aminoglycoside-resistance patterns of the strains to 16 different aminoglycoside antibiotics. The predominant mechanisms of resistance found were APH(2") + AAC(6') production in staphylococci. AAC(2') production in Providencia spp., and AAC(3) production in the other Gram-negative genera.