Brownish-black concentrates of Pl. vivax were readily obtainable from 500 ml samples of heavily infected blood by the method of lysis and differential centrifugation described above. In general, purity was highest and the yields best when the parasites consisted mainly of large, pigmented forms, and in one such instance the nitrogen content of the product was only 0.4 mg per billion parasites.

Large, pigmented forms of Pl. vivax and Pl. malariae are also slowly separable from unparasitized red cells in 5 to 25 ml blood samples in a strong, unsymmetrical magnetic field, but the method does not appear suited to the preparation of large quantities of plasmodia.

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The work described in this paper was carried out under a contract recommended by the Committee on Medical Research, between the Office of Scientific Research and Development and Columbia University, Dec. 1942–Dec. 1945. The work was also carried out in part under the Harkness Research Fund of the Presbyterian Hospital in New York City.

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