Proteinuria may be associated with hypertension and progression of renal insufficiency, which in turn may accompany abnormalities in cell calcium homeostasis. Therefore, urine from rats made proteinuric by puromycin aminoglycoside administration was analyzed, in a search for factors affecting cellular calcium transport. Proteinuric urine was fractionated by thin-layer chromatography and HPLC, and the effects of the fractions on the plasma membrane calcium pump in human red blood cells were assessed. Proteinuric urine contained a powerful specific inhibitor of the calcium pump that had little or no effect on the Na+/K+- or Mg2+-ATPases. The inhibitor was characterized as a neutral lipid, migrating as a single band, that inhibited 45Ca2+ efflux. To confirm the presence of an inhibitor in other proteinuric states, the urine from two patients with proteinuria was examined and subjected to chromatography as in the rat studies. These thin-layer chromatographic fractions contained a very strong inhibitor of the red blood cell calcium pump, suggesting that this substance may have relevance for the pathogenesis of proteinuric renal disease in human patients. Rat proximal tubule cells in tissue culture, when challenged with lipid-replete albumin, secreted an inhibitor of the calcium pump that migrated in the same chromatographic band as the urine factor. Therefore, the processing of fatty acids borne by albumin into endocytosing proximal tubular epithelium results in the synthesis and release of a previously unknown lipid modulator of the calcium pump, an effect that may predispose kidney tissue toward elevations in cytosolic calcium levels in target cells.