ZEITZ, L., CANADA, T. R., DJORDJEVIC, B., DYMBORT, G., FREEMAN, R., MCDONALD, J. C., O'NEIL, J., AND LAUGHLIN, J. S. A Biological Determination of the Variation of Fast-Neutron Field Quality with Depth, RBE, and OER. Radiat. Res. 63, 211-225 (1975). The biological effectiveness of two cyclotron-produced fast neutron fields (E, - 3.5 and 7 MeV) has been studied as a function of depth within a water phantom. Aerobic and hypoxic HeLa S3 cell survival data were obtained at depths of 2 and 12 cm and compared with those measured in a 1"Co gamma ray field. Although the resulting survival curves show a small decrease in sensitivity with increasing depth, the oxygen enhancement ratio remains constant and has a value of 1.8 +- 0.1 in both fields.