A cell-free factor, "RLP," derived from sheep spleen, which is capable of inhibiting radiation leukemogenesis in mice, has been shown to be heat-labile, nondialyzable, not present in significant amounts in the RNA fraction of the crude material, not inactivated by DNAase, and recoverable in active form after precipitation with 30 per cent ammonium sulfate. From the evidence so far available, it thus appears that "RLP" is a protein of high molecular weight.

Only borderline inhibition was obtained with extract of sheep kidney, and no inhibition with lactalbumin hydrolysate, thus supporting the view that the inhibitory effect of sheep spleen extracts is probably the result of a specific action and not brought about by a nonspecific, "stress-mediated" mechanism.

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© 1965 by American Society of Hematology, Inc.
1965
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