1. Leucocyte sodium transport was investigated as a possible assay for the small molecular weight natriuretic material isolated from the urine of normal subjects who had undergone volume expansion by saline infusion.
2. This fraction (fraction four or FIV), inhibitory to sodium transport in several other assays, was also found to inhibit leucocyte sodium transport.
3. FIV isolated from the urine of five normal subjects undergoing the natriuresis of mineralocorticoid ‘escape’ was also found to be inhibitory to leucocyte sodium transport.
4. Leucocytes isolated from the blood of the same subjects during mineralocorticoid escape showed decreased sodium transport.