Urinary excretion of albumin, free light chains of immunoglobulins and beta-2-microglobulin was measured after injection of certain amino acids and derivatives. Substances with a positively charged group located terminally in the molecule (ornithine, lysine, arginine, and the almost not metabolized epsilon-amino-caproic acid, and Cyclocaprone) proved to inhibit instantaneously the tubular protein reabsorption. Lysine is the most effective molecule tested. We hypothesize that the initial event in the normal tubular protein reabsorption is a binding between a free positive amino- or guanidino-group in the protein molecule and a negative site on the tubular cell surface. In an attempt to achieve complete inhibition of tubular protein reabsorption, large doses of lysine were injected intravenously. The maximal protein clearance rose from the exceedingly low baseline excretion to values probably representing near complete inhibition of protein reabsorption. The minimal value of albumin concentration in the glomerular filtrate was calculated to 2.6 mg/l.