When added to human leukemic leukocytes in vitro, 4-amino-pteroylglutamic acid (Aminopterin) produced a marked inhibition of mitosis in low concentrations. Attempts to counter its action by means of folic acid gave varying results. The mechanism of the observed effect is discussed.

ACKNOWLEDGMENTS The author wishes to acknowledge gratefully the valuable advice given by Dr. E. Friedmann during the work leading to the preparation of this paper and the financial support provided by the Saltwell Trustees.