Purpose: The aim of this double-blind, randomised cross-over study was to compare the antibacterial effect and the substantivity of two toothpaste formulations containing amine fluoride (AmF) or zinc chloride (ZnCl2). Materials and Methods: After a professional tooth cleaning, 20 volunteers refrained from all oral mechanical hygiene measures for the subsequent 24 hours (day 0). Subsequently, a plaque sample was taken from three teeth and analysed for vitality of the plaque bacteria by means of the vital fluorescence technique (VF0; in %). After assessment of this baseline value the subjects had to brush their teeth for 2 minutes with 1.2 ml of the allocated toothpaste containing (a) 0.66% AmF or (b) 0.2% ZnCl2. For the following 8 hours no oral hygiene measures were allowed. After 4 and after 8 hours further plaque samples were analysed for biofilm vitality (VF4, VF8). During the following 3 days the volunteers had to brush twice daily for 2 minutes with the allocated toothpaste. On day 4, plaque index was assessed using the criteria of Quigley and Hein (Turesky modification). After a washout time of 9 days the next test cycle with the other toothpaste was started. Results: Both toothpastes reduced the biofilm vitality significantly at VF4 and VF8 compared with VF0 (p = 0.001). While after 8 hours the vitality values for the ZnCl2-toothpaste obtained significantly higher reductions (53%) than for the AmF-toothpaste (44%), results for plaque index were not significantly different (0.98 and 1.04 respectively). Conclusions: Both toothpastes showed a significant and prolonged antibacterial effect up to 8 hours with a benefit in favour of the ZnCl2 toothpaste. Palabras clave: amine fluoride, antibacterial agents, substantivity, toothpastes, zinc chloride