A four variable constitutive equation was developed utilizing the method first presented by Schneck and Walburn. Spearman rank correlation coefficients were calculated on whole blood samples within a narrow range of hematocrit to investigate further the effect of the various plasma constituents on whole blood viscosity. Viscosity measurements were made on one hundred anticoagulated blood samples of known hematocrit and chemical composition. The constitutive equation was developed using a power law functional form similar to that employed by Schneck and Walburn. This equation contains two parameters, the consistency index and the non-Newtonian index. A computerized multiple regression technique with apparent viscosity as the dependent variable was used to determine the particular form of these parameters. The one, two and three variable models developed confirmed the results of the previous work of Schneck and Walburn. The four variable model included the total lipids in combination with the concentration of total protein minus albumin and hematocrit. Spearman rank correlation coefficients showed the highest correlations between whole blood viscosity and the plasma constituents to be those of the globulins, total protein and fibrinogen. The constitutive equation developed did not show as high a correlation between experimental data and theory as did the Schneck-Walburn three variable model. The addition of a fourth variable did produce a statistically significant increase over the best three variable model of the present study. / M.S.