The susceptibility in vitro of 28 Corynebacterium group D2 strains, mainly isolated from urine, to fifteen antimicrobial agents and acetohydroxamic acid (AHA) was determined at two pH values. The bactericidal activity of four antimicrobials and AHA was studied in three reference strains in broth at two pHs and with two inoculum sizes. The activity of norfloxacin and AHA, against one selected strain, in broth and human urine, was also determined. Vancomycin, ofloxacin and norfloxacin were the most active agents tested. Norfloxacin acted bactericidally in broth and in human urine but was not synergic with AHA.