In order to compare the Azure A Assay to other commonly accepted measures of heparinization, samples of fresh donor sheep blood were prepared with known levels of heparin between 0 and 9 U/ml. For each sample, two determinations of the Activated Clotting Time (ACT) and Whole Blood Partial Thromboplastin Time (WBPTT) were performed. Similarly, an Azure A Assay standard curve of absorbance versus heparin concentration was constructed for the same blood. Three replicates of the procedure were performed, and the average relative error in each assay was computed as the 95% confidence interval on the regression line divided by the slope of the regression curve. Parallel studies were also performed to assess the effect of various plasma constituents on the Azure Assay. Results indicate that all of the assays are equally accurate for heparin levels at or below 2 U/ml, while the ACT and Azure assays are equivalent at heparin levels near 4 U/ml, but the WBPTT is not usable at concentrations above 2 U/ml. The Azure Assay is not sensitive enough for very low heparin levels (less than 1 U/ml), but it is quite accurate and, indeed, is the only rapid assay for heparin concentrations higher than 4 U/ml. In the sensitivity studies, variations in albumin levels and plasma platelet count were shown to have a significant effect on the accuracy of the Azure assay, whereas calcium levels had no effect.