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A detailed study of the Akerfeldt test for plasma oxidase activity has been carried out and it has been found that: 1. The optimum activity of the enzyme system involved occurs at pH 5.5-6.0. 2. The amount of plasma needed for the test at pH 6.0 can be reduced to 0.2 ml. 3. The reduced amount of plasma removed the inhibitory influence of albumin and a linear response was always obtained. 4. The inhibitory effect of albumin and glutathione is due in part to inhibiting the enzyme system, and in part to the reduction of oxidized DPP. 5. The inhibiting influence of Versene appears to be due to the chelating and inactivation of heavy metal catalysts. 6. The effect of ascorbic acid is solely one of increasing the lag period without inhibiting the enzyme system. As a result of this study a modification of the Akerfeldt test is proposed. With this proposed method it was determined that the plasma oxidase activity, ACA, of 77 normal individuals was 8.0 ± 2.8 units. 4101 Woolworth Ave. Omaha 5, Neb.