The following equation for hepatic "clearance" has been derived which is comparable to the equation used for renal clearance: C/V = (P - P)[image], where C = vol. of plasma cleared in 1 hr., V = vol. of extra-cellular solvent, P = initial plasma concn., and P[image] = plasma concn. after 1 hr. It is assumed that the geometric mean of the 2 plasma concns. is equal to the true mean. To avoid determining plasma vol. and corrections for surface area, the expression, C/V, is kept intact and referred to as fractional clearance. Other intervals than 1 hr. may be used by correcting with appropriate factors. This equation is adequate for clinical use, and retention and tolerance data already at hand may be recalculated and used for direct quantitative comparisons. The following more precise formula may be used when warranted by highly accurate technics: C/V = (log P - log P[image])/ t log e.