Transmembrane electrical and pH gradients have been measured across human erythrocytes and peripheral blood lymphocytes using equilibrium distributions of radioactively labelled lipophilic ions, and of weak acids and weak bases, respectively. The distributions of methylamine, trimethylamine, acetic acid and trimethylacetic acid give calculated transmembrane pH gradients (pHe‐pHi) for erythrocytes of between 0.14‐0.21 for extracellular pH values of 7.28‐7.16. The distributions of trimethylacetic acid, DMO and trimethylamine were determined for lymphocytes, establishing upper and lower limits of the calculated pH gradient over he external pH range of 6.7 to 7.7. Tritiated triphenylmethyl phosphonium ion (TPMP) and 14C‐thiocyanate ion (SCN) equilibrium distributions were measured in order to calculate transmembrane electrical potentials, using tetraphenylboron as a catalyst to facilitate TPMP equilibrium. Transmembrane potentials of −7 to −10 mV were calculated from SCN and TPMP, respectively for red cells, and −35 to −52 mV respectively, in the case of lymphocytes. Distributions of TPMP and potassium ions were determined in the presence of valinomycin over a wide range of extracellular potassium concentrations for red cells and the calculated Nernst potentials for TPMP compared to the calculated potential using the Goldman equation for chloride and potassium ions. Distributions of TPMP, SCN and potassium ions were also determined for lymphocyte suspensions as a function of extracellular potassium and the calculated Nernst potentials for TPMP and SCN compared to the calculated potassium diffusion potential.