A comparative assessment of toxicity of magnetite (Fe3O4) particles within the nanometric (10 nm and 50 nm) and micrometric (1 nm) ranges was conducted basing on shifts in the cell composition of the bronchioalveolar lavage liquid 24 h after intratracheal administration of these particles as well as on a number of indicators of the organism status after repeated intraperitoneal injections of the same materials. Mass doses being equal, nanoparticles show a significantly more expressive biological aggressiveness than particles within the micrometric range, but induce a more active and effective protective response of alveolar phagocytocis. The relationship between the diameter and absorptive toxicity of particles is ambiguous within the nanometric range which could be linked to toxicokinetic differences controlled both by physiological mechanisms and direct penetration of nanoparticles through biological barriers and eventually by different solubility.