Megaloblastic changes in the erythrocytic series and cellular giantism in the myeloid line characterize the bone marrow response to 5-fluorouracil in man.

Cytoplasmic vacuolizations occur in reticulo-endothelial cells, megakaryocytes, and myeloid elements.

During development of these morpholigic changes in marrow, associated cellular giantism, polynucleism, and nuclear pattern changes appear in the oral, vaginal, colonic mucosa, and in some tumors resembling those encountered in the bone marrow.

The myeloid-erythroid ratio was found to be useful for the purpose of identifying dosage end-points.

The physiological implications Of these changes as related to nucleic acid metabolism were discussed.

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