The minimum inhibitory concentrations (MICs) of 11 aminocyclitol antibiotics were determined for 773 recent clinical isolates from infected hospital patients, most of whom were immunocompromised. There was a bias towards aminoglycoside-resistant organisms, and 21% of Pseudomonas aeruginosa isolates were resistant to gentamicin. The antibiotics included the natural agents gentamicin, tobramycin, sissomicin and kanamycin and the semi-synthetic compounds amikacin, netilmicin and dibekacin with some newer agents, including O-demethyl fortimicin, Hapa gentamicin B and 5-epi-sissomicin, not available commercially. Tobramycin, sissomicin and gentamicin had similar spectra with tobramycin more active against Ps. aeruginosa, sissomicin more active against proteus and gentamicin more active against serratia. The differences in spectrum between amikacin and netilmicin were marginal, reflecting the relatively low prevalence of acetyltransferase-producing organisms in our collection. Hapa-gentamicin B was the most active aminoglycoside against staphylococci and indole-positive proteus, netilmicin against Escherichia coli and 5-epi-sissomicin against Ps. aeruginosa. Overall 5-epi-sissomicin was the most active aminoglycoside tested. Both Hapa-gentamicin B and 5-epi-sissomicin have potentially valuable antibacterial spectra which merit clinical studies.