Improvements with regard to the in-vitro activity of new macrolides are marginal and apply mainly to Haemophilus spp., Moraxella catarrhalis and Neisseria gonorrhoeae (e.g. azithromycin is two to eight times more active than erythromycin) and to non-enterococcal streptococci (e.g. clarithromycin is two to four times more active than erythromycin). The increase in activity against staphylococci is even less striking, being restricted to a few species and limited to clarithromycin (twice as active as erythromycin). The Enterobacteriaceae, as well as glucose non-fermenting Gram-negative bacilli, remain outside the therapeutic range of the new macrolides, as they were for the established compounds. The majority of enterococci and Corynebacterium jeikeium are resistant to all macrolides, whereas Corynebacterium diphtheriae is highly susceptible. In-vitro susceptibilities both of Campylobacter jejuni/coli and Helicobacter pylori indicate only moderate susceptibility to macrolides and the azalide. In the case of anaerobic organisms, clarithromycin is the most active macrolide against the majority of species. Dirithromycin, clarithromycin, roxithromycin and josamycin, and the azalide azithromycin, are similar in their antibacterial spectrum to erythromycin. New macrolides differ from established compounds largely in their pharmacokinetic behaviour and only minor progress has been achieved in improving their antibacterial spectrum.