Renal uptake of 99mTechnetium sulphur colloid (99mTcSC) was investigated in 99 animals with experimentally induced unilateral pyelonephritis and in 60 animals with a thermally induced renal injury. The degree of localization of 99mTcSC in the kidney was determined by direct counting of retained isotope after bilateral nephrectomy at autopsy. Phagocytosis of 99mTcSC by inflammatory tissue has been reported and it was anticipated that renal infection would lead to an increase in 99mTcSC uptake by the kidney. In fact it was found that renal infection with pathologic changes resulted in a significant decrease in the localization of 99mTcSC in the pyelonephritic kidney. A similar decrease was found in the thermally injured kidney but renal infection without pathologic changes did not give rise to any changes in radionuclide uptake. These findings suggested that phagocytosis by the proximal convoluted tubular cells is the normal mechanism for 99mTcSC accumulation in the kidney and that renal infection reduced the phagocytic capability of these cells.