Intensive use of anticholinergic pesticides such as organophosphate esters and threat of chemical warfare establish the need for rapid, high throughput, reliable and standardizable determinations of blood cholinesterase levels to provide early warning of exposures to neuroactive chemicals. Many clinical and research laboratories use the colorimetric Ellman assay based on hydrolysis of acerylthiocholine. CHPPM (US Army Center for Health Promotion and Preventive Medicine) uses a dower method, the delta pH based on that of Michel to monitor approximately 25, 000 DOD personnel annually. One of the goals of this project is to establish a conversion factor between the pH and colorimetric assays applicable to monitoring studies and field tests. Blood drawn under the appropriate regulations by CHPPM was centrifuged and detergentlysed before being subjected to the delta pH assay with acetylcholine as substrate. Duplicate RBC samples were sent to UC Davis to be assayed with acerylthiocholine by the Ellman method. Samples were lysed, diluted with buffer and run with and without quinidine to separate activities due to acetylcholinesterase and non-specific cholinesterases. For example, slopes of delta pH vs Ellman for three of five sets of samples yielded r2 correlations of 0.74 to 0.8. Comparisons continue to establish critical assay conditions and Ellman equivalents of me delta pH assay.