Ponsinomycin is a new macrolide antibiotic. Its effect on DHE pharmacokinetics was investigated in this study. Twelve young healthy volunteers received a single 9 mg oral dose of DHE before and on the 8th day of treatment (800 mg twice daily) with ponsinomycin. DHE was assayed in plasma by RIA. Because of low plasma levels, only peak concentrations could be accurately compared for a ponsinomycin effect. We observed a 3-40-fold increase in maximum DHE plasma levels in the majority of cases and a much more important effect on one occasion, when DHE was administered in the presence of ponsinomycin. These data are consistent with an increase of DHE bioavailability in the presence of ponsinomycin, probably related to a reduction of its first-pass elimination. This pharmacokinetic interaction is likely to have clinical consequences and administration of ponsinomycin should be avoided in patients treated orally with DHE.