Macrophage Large phagocytic cells of the blood or lymph systems that can engulf particles or small organisms. Mechanism of Action The specific manner by which a substance causes a particular effect. Metabolism The conversion of a chemical from one form to another.  It is also known as biotransformation. Metabolite A chemical produced when a substance is metabolized by a biological organism. Mg/kg A commonly used dose that stands for mg of a substance per kg of body weight. Mg/kg/day A commonly used dosage that stands for mg of a substance per kg of body weight on a daily basis. Mg/M3 An exposure unit used to express concentrations of particulates in the air, standing for milligrams of compound per cubic meter of air. Microgram (µg) A commonly used unit of weight consisting of one millionth (1 x 10-6) of a gram. Microsomes The subcellular organelles that are a part of the smooth endoplasmic reticulum. Milligram (mg) The most commonly used unit of measure in medicine and toxicity consisting of one thousandth of a gram (1x10-3 g). Monooxygenase Enzyme system (such as cytochrome P450 ) involved in the oxidation of compounds.