Among the allergic accidents due to drugs, three clinical syndromes with brutal onset and acute evolution can occur. They are thrombocytopenic purpura, agranulocytosis and hemolytic anemia. An association of two or three syndromes is rare. The occurrence of these cytopenias resulting from an immuno-allergic process is due to a previous sensitization caused by a drug which, in the course of later treatment, would be able to induce an hematological accident in these sensitized subjects. These allergies always involve the formation of serum antibodies in sensitized subjects, the specific activity of which appears on the different cellular types in the presence of the drug responsible or of chemically related products. The specificity of allergic antibodies has been studied. Cross-reactions of these antibodies against products with a chemical formula close to that of the drug responsible have been observed. The physiopathological mechanisms of immuno-allergic sensitization are still uncertain. The most probable hypothesis involves the fixation of soluble complexes on to target cells. Further complement fixation would lead to a possible protection against these cytopenias.