Aims: A plethora of drugs produce adverse cardiac side effects by inhibition of the HERG channel. Drug effects have been mainly studied using the heterologously expressed erg1a isoform. Since recent studies suggest that native cardiac erg channels also involve the isoform erg1b, we have now investigated the effects of the macrolide antibiotic roxithromycin on heteromeric erg1a/1b channels. Methods: CHO cells were coinjected with rat erg1a and erg1b cDNA (ratio 1:1) and erg currents were recorded with the patch-clamp technique in conventional or perforated-patch whole-cell configuration at room temperature. Results: Roxithromycin induced a concentration-dependent inhibition of erg1a/1b currents with a half maximal effect at about 100 mM. At this concentration, roxithromycin also slightly shifted the voltage dependence of erg1a/1b channel activation to less negative potentials and markedly slowed deactivation kinetics. In contrast, activation and inactivation kinetics were accelerated by roxithromycin, which also induced more complete inactivation at higher depolarizations. These combined effects resulted in a pronounced inhibition of steady state erg1a/1b current at more depolarized potentials. The presence of roxithromycin did not prevent a complete block of erg1a/1b current by additionally applied 10 mM terfenadine. Different from erythromycin, the effects of roxithromycin were not enhanced at elevated temperature. Conclusion: Comparison with published data suggests a similar or slightly lower sensitivity to roxithromycin of heteromeric erg1a/1b channels compared to homomeric erg1a (HERG) channels. Thus, the respective EC50 values for erg1 channel inhibition are clearly higher than reported plasma concentrations of roxithromycin achieved with normal oral doses. Nevertheless, special care should be taken by the simultaneous use of different drugs resulting in additive erg channel block.