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In humans, clinically relevant drug–drug interactions occur with some macrolide antibiotics via the formation of stable metabolic intermediate (MI) complexes with enzymes of the cytochrome P4503A (CYP3A) subfamily. The formation of such complexes can result in a decreased biotransformation rate of simultaneously administered drugs. In previous studies it was shown that the veterinary antibiotic tiamulin was also able to form a stableMIcomplex in pigs and rats. In the present study the relative CYP3A inhibiting potency andMIcomplex formation of a series of macrolide antibiotics and tiamulin were studied in microsomal fractions of goat and cattle and in a cell-line expressing bovine CYP3A. Tiamulin and triacetyloleandomycin (TAO) were found to be effective inhibitors of CYP450 activity in all systems tested. Erythromycin and tilmicosin were found to be relatively less effective inhibitors of CYP450 activity in microsomes, and their activity in the bovine CYP3A4 expressing cell line was relatively weak. Tylosin was shown to be a weak inhibitor in microsomes and not in the cell line, whereas spiramycin had no effect at all.MI-complex formation measured by spectral analysis was seen withTAO, tiamulin, erythromycin and tylosin, but not with tilmicosin and spiramycin. Although additional factors play a role in vivo, these results may explain potential drug–drug interactions and differences between related compounds in this respect.