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Very rapid production in vitro of a particular type of hematin (B-hematin) with high yield at physiological pH and temperature is presently reported, using a simple chemical system including an organic acid in the absence of any enzyme. B-hematin is characterized by infrared spectra and solubility properties. The antimalarial drug chloroquine practically prevents formation of this hematin under the conditions used. It has recently been suggested that the purified malaria pigment hemozoin is identical with beta-hematin, a form of ferriprotoporphyrin IX sparingly soluble in different solvents which is prepared under non-physiological conditions. Relations between B- and beta-hematin and possible mechanisms of hemozoin formation and antimalarial drug action, are discussed.