inhibitors, there is little doubt that it would be great. It is thus possible that with a suitable choice of inhibitors the I50 ratio method could be used to establish, first, whether the whole of the activity towards one substrate is identical with the activity towards another and, secondly, whether any part of it is.So far, a purified preparation of the DFP-insensitive enzyme has not been obtained, and it is thus impossible to be certain of its exact specificity. It seems unlikely, however, that it can contribute to the ACh hydrolysis of plasma in view of the total inhibition produced in the unpurified preparation by very low DFP concentrations. SUMMARY 1. Using human plasma as a source ofa single nonspecific esterase and the cholinesterase inhibitors di-i8opropyl fluorophosphonate (DFP) and di-(2-chloroethyl)methylamine hydrochloride (DDM), a-method is described for characterizing the enzymic activity of the unpurified plasma towards representative choline and non-choline esters. This involves the determination for each substrate of the ratio of the concentrations of each inhibitor which produce 50 % inhibition of hydrolysis (the DDM: DFP I$o ratio). The method enables the most probable value of the ratio and the range of probable values to be estimated.2. The values for the I5 ratio for the different esters are closely similar, ranging from 2-28 to 2-44 x 105.3. The limitations of the method, and its possible wider application, are discussed.4. Evidence has been obtained for the existence in human plasma of a small amount of a second enzyme, DFP-and DDM-insensitive, whichaccounts for 5-20% (depending on the substrate) of the aliphatic esterase activity of the plasma, and which also hydrolyzes triolein.5. When the h50 ratios for tributyrin and triacetin are corrected for the contribution of the DFPinsensitive esterase they become identical with the value (2.28 x 105) for acetylcholine.

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Although concentrations of nucleic acids have been determined by various workers in some animal embryonic organs, reports on similar studies in human material are scarce. This communication describes some biochemical investigations in regard to nucleic acid and protein concentrations in various organs and membranes of human fetuses obtained by therapeutic abortion. The pattern of nucleic acid and protein concentrations at various stages of the first half of gestation appears to be characteristic of each organ. The recorded results are related to known embryological data such as the differential rates of development in individual organs, and the relevant biochemical aspects of human embryonic and fetal growth are discussed.

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1. L‐histidine was given intravenously and orally to pregnant and non‐pregnant women, and was estimated in the blood by a modified diazo reaction and by its reaction with bromine in the urine. 2. On intravenous as well as on oral application, L‐histidine disappeared only very slowly from the blood‐stream of pregnant women as compared with non‐pregnant women, large amounts of it being simultaneously excreted in the urine. This observation does not confirm the results of Page, who found that on intravenous injection L‐histidine leaves the blood‐stream of pregnant women at least as quickly as that of non‐pregnant persons. It seems, moreover, to support the previously suggested theory of a reduced activity of histidase in human pregnancy. 3. In severe toxæmia of pregnancy no histidine could be detected in the urine even after the injection or ingestion of histidine although the blood levels remained high. This indicates a retention of L‐histidine by the kidney. 4. The results obtained in mild pregnancy toxæmia were intermediate between those found in severe toxæmia and in normal pregnancy.

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