Context: This research aimed at verifying the efficacy of homeopathic treatments by plant-based bioassays, which may be suitable for basic research, because they lack placebo effects and provide large datasets for statistical analyses.Objective: To evaluate the effects of homeopathic treatments of arsenic trioxide (As2O3) on tobacco plants subjected to tobacco mosaic virus (TMV) inoculation as biotic stress.Design: Blind, randomized experiment using tobacco leaf disks.Materials and methods: Tobacco plants (Nicotiana tabacum L. cultivar Samsun) carrying the TMV resistance gene N. TMV inoculated leaf disks were floated for 3 days in the following:• Distilled water (control)• H2O 5 and 45 decimal and centesimal potencies• As2O3 5 and 45 decimal and centesimal potenciesThe main outcome measures is the number of hypersensitive lesions observed in a leaf disk.Results: Homeopathic treatments of arsenic induce two effects on the plant: (i) increased resistance to TMV; (ii) decrease variability between experiments (system variability).Conclusions: In this experimental model two actions of homeopathic treatment were detected: decrease in system variability and enhancement of the natural tendency of the system towards an ‘equilibrium point’.