We studied the in-vitro activity of HMR 3004 (RU 64004), a new ketolide, against 161 clinical isolates of Corynebacterium spp. including isolates resistant to erythromycin A, josamycin and lincomycin. HMR 3004 was active against all erythromycin A-sensitive isolates as well as against 75.8% and 45.4% of erythromycin A-intermediate and -resistant isolates, respectively. In contrast, HMR 3004 was active against 40 (46.5%) of 86 isolates resistant to erythromycin A, josamycin and lincomycin as well as against two isolates that were resistant to erythromycin A and lincomycin but not resistant (i.e. susceptible or intermediate) to josamycin.