Blood is centrifuged in tubes drawn out to sealed graduated and calibrated capillaries. The volume of erythrocytes is noted and they are suspended in various salt solns. for 5 min. and recentrifuged. The pH of the soln. is measured, the change in erythrocyte volume noted, and the cells analyzed. If erythrocytes take up X m.eqs. Cl- from pure KCl solution in 5 min. at a given pH, they take up approximately X m.eqs. of Cl- and B- from a soln. containing equivalent amts. of KCl and KB, at the same pH. If Cl- taken up from KC1 and KB is Y m.eqs, cell B- is X [long dash]Y m.eqs. The relative ratios [(X-Y)/Y], of B-/C1-, C-/Cl-, D-/C1-, are used as an index of the relative permeation rates of B, C, D, etc. The permeation rates of inorganic anions and CNS-follow a lyotropic series CNS > I > NO3, Cl > SO4 PO4. In the organic compounds, rapidly permeating acids are soluble in organic solvents, weakly dissociated and surface active. Permeation therefore varies directly with the length of the carbon chain and with pK, and inversely with the number of polar groups. Oxalate is exceptional in permeating with great rapidity. The permeation of organic acids relative to Cl- is greater at low pH.