The in vitro activity of dirithromycin (LY-237216), a new macrolide erythromycin derivative, was compared to that of four other agents (clarithromycin, erythromycin, roxithromycin, clindamycin) against 334 gram-positive isolates obtained from cancer patients. Dirithromycin was similar in potency and antimicrobial spectrum to the other agents tested. It was very active against beta-haemolytic streptococci and Streptococcus pneumoniae, and moderately active against penicillin and methicillin susceptible Staphylococcus aureus, Bacillus spp., Listeria monocytogenes and Corynebacterium jeikeium. Erythromycin resistant organisms were also resistant to dirithromycin.