The use of medicinal plants by the general population is an old and still widespread practice, which makes studies of their genotoxicity essential. Psidium guajava L. and Achillea millefolium L. are examples of plants commonly used in popular medicine. P. guajava L. is indicated for diarrhea and also as an antiseptic, while A. millefolium L. is indicated as an analgesic, antispasmodic, digestive, diuretic, antiseptic, astringent, emollient, wound healer and hemorrhoid medication. The aim of this study was to determine the effects of the infusions of these two plant species on chromosomes and the cell cycle. Leaves from the plants were used to prepare infusions, in the same manner as teas, but at two different concentrations. Allium cepa L. root-tip cells (P. guajava L. - 2.62 and 26.2 mg/mL, and A. millefolium L. - 3.5 and 35.0 mg/mL) and Wistar rat bone marrow cells (P. guajava L. - 2.62 and 26.2 mg/100g body weight, and A. millefolium L. - 3.5 and 35.0 mg/100g body weight) were used as in vivo plant and animal test systems, respectively. Human peripheral blood lymphocytes (P. guajava L. - 0.262 and 2.62 µg/mL culture medium, and A. millefolium L. - 0.35 and 3.5 µg/mL culture medium) were used as in vitro test system. The P. guajava L. infusion at the higher concentration caused a statistically significant inhibition of cellular division in the onion root-tip cells, not observed in onion root-tip cells treated with A. millefolium L. No statistically significant alterations were found, as compared to untreated controls, in either the cell cycle or the number of chromosome alterations, after treatments with either plant, in rat cells or in cultured human lymphocytes. These results regarding the cytotoxicity and mutagenicity of these plants provide valuable information about the safety of using them as therapeutic agents.