Serum level tests were carried out on healthy cows with a combination drug of one part Berenil and two parts Reverin. When compared with the commercial preparations Berenil (Diminazene) and Reverin (Rolitetracycline), the kinetic behaviour of Reverin in the combination preparation was identical. In the case of Berenil, administered in the form of the combination drug, the second slow elimination phase with a 63 hour half-life, as found after the administration of Berenil only, could not be observed. Eight hours after administration of the combination only minimal Berenil serum levels were detectable, after 24 hours the levels were lower than the limit of detection. This has a practical bearing on the question of residues.