The possibility of separating ascites tumor cells with different numbers of chromosomes by means of counter-streaming centrifugation has been studied with the hyperdiploid Ehrlich ascites tumor (ELD) and its hypertetraploid offshoot (ELT).

From ELD a fraction significantly higher in deoxyribonucleic acid phosphorus (DNAP) per cell than the unfractionated suspension was obtained. Inoculated into a new mouse this fraction developed into a suspension which was invariably lower in DNAP per cell than the inoculum. Repeated inoculation of this fraction followed by further fractionation increased the mean number of chromosomes per cell, giving about twice the original number.

When another fraction of ELD, which is low in DNAP, was inoculated, the tumor suspension produced showed the same value of DNAP as that of the inoculum.

Fractionation of original ELT gave two fractions high in DNAP per cell, one composed chiefly of octoploid cells, and another of tetraploid and octoploid cells.

The distribution of the different types of tumor cells upon the three fractions obtained with counter-streaming centrifugation is not in keeping with that which should be expected from their volumes and contents of DNA.

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This investigation was supported by a grant from the Swedish Cancer Society, which is gratefully acknowledged. I am very much indebted to Professor G. Klein, Karolinska Institutet, Stockholm, for material and advice, and to Professor A. Levan, Lund, for valuable information on the chromosome numbers of some fractionated material. I thank Mr. P.-Å. Lindström and Miss Gertrud Thelin for skilful technical assistance.

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