Causes and mechanisms of hematotoxicity are reviewed. The formation and differentiation of blood cells are discussed. The formation and function of red blood cells, platelets, and white blood cells are summarized. Disorders of these blood components such as anemia, thrombocytopenia, and granulocytosis are briefly explained. Principles of oxygen transport are presented in a summary of the components and function of hemoglobin. Hypoxia refers to any condition in which there is an inadequate supply of oxygen to the tissues. The mechanisms of selected chemicals which interfere with normal oxygen transport by hemoglobin are discussed. Carbon-monoxide is the best known chemical agent that interferes with such transport and may produce asphyxial hypoxia. Carbon-monoxide has a stronger binding affinity for hemoglobin than oxygen, thereby forming carboxyhemoglobin. The lethal concentration of carbon-monoxide is small. Nitrites, aromatic amines, nitro compounds, and chlorate salts oxidize the iron in hemoglobin to form methemoglobin. Sulfhemoglobin and Heinz bodies are included as byproducts of hypoxia. Sulfhemoglobin is a mixture of oxidized and denatured hemoglobin. Heinz bodies are clumps of unnatured hemoglobin bound to the red cell membranes. Some chemicals that produce Heinz body anemia are chlorates, naphthalene (91203), arsine (7784421), phenylhydrazine (100630), and methylene-blue (61734). Cytotoxic hypoxia results from an interference of the utilization of oxygen during cell metabolism in the presence of an adequate oxygen supply and blood flow. Chemicals that cause this condition include cyanide (57125) and hydrogen-sulfide (7783064) which are both potent inhibitors of cytochrome oxidase. Chemically induced blood disorders noted are dyscrasias, thrombocytopenia, agranulocytosis, aplastic anemia and pancytopenia, and hemolytic anemia. The analysis of hematotoxicity includes hematocrit, hemoglobin measures, red cell count, white cell count, and platelet count. The author concludes that chemicals that are toxic to blood elements or reduce them are numerous and diverse in mechanism.