Materials and Methods.-An ascites tumor, originally a lymphosarcoma, which has been in use in our laboratory for several years, 4 was used as the tissue for these studies. Following intraperitoneal inoculation of 0.1 ml of ascitic fluid from a tumor-bearing mouse into C3H mice, the tumor grows rapidly, yielding 2-4 ml of ascitic fluid containing about 50 per cent cells on the sixth to the eighth day. The animals were sacrificed and several tumors pooled to obtain the desired volume. A solution containing P32 as orthophosphate was added, such that the final concentration was 50, uc/ml. Tumor cells, with P32 added, could be maintained for several hours in an ice bath with no significant incorporation of isotope into nucleic acid and without loss of synthetic capacity when subsequently incubated at 370C. The tumor fluidwas transferred in volumes of0. 4 ml into tubes containing0. 1 ml of saline with and without the inhibitor under study. The preparation was then incubated for the desired length of time at 370 with gentle shaking. The reaction was stopped by transferrring the tubes to an ice bath and adding 2.5 ml of 5 per cent trichloro-acetic acid.

The preparations were homogenized in Potter-Elvehjem tubes and centrifuged for 10 min at 600 g, following which a portion of the supernate was transferred to an ashing tube for subsequent evaluation of the acid-soluble phosphorus. The acid-insoluble residuewas washed five times in cold 5 per cent trichloroacetic acid, and the nucleic acids extracted in a boiling water bath in 10 per cent NaCl buffered to pH 7.8. The nucleic acids were precipitated from the extract with two volumes of 95 per cent ethanol, the RNA was hydrolyzed with 0.1 N NaOH, and the DNA re-precipitated with trichloroacetic acid and hydrolyzed by methods previously described. 4 All samples-acid-soluble, RNA, and DNA-were wet-ashed, diluted to volume, counted with an end-window Geigertube, and analyzed for phosphorus content. The activity of each sample was calculated as counts per minute per microgram of phosphorus per milliliter, and the activity of the DNA and RNA was then divided by the acid-soluble precursor pool activity to give the relative specific activity of DNA and RNA. In evaluating the effect ofthe inhibitor the relative specific activity of a treated sample is compared to that inuntreated controls and is referred to as the comparative specific activity, expressed as per cent of control. Results.-Table 1 indicates the relationship of concentration of hydroxyurea to incorporation of P32 into DNA and RNA. Almost 90 per cent inhibition of DNA 1033