The role of the albumin-carried fatty acids in the induction of tubulointerstitial injury was studied in protein-overload proteinuria. Rats were injected with fatty acid-carrying BSA [FA(+)BSA], fatty acid-depleted BSA [FA(−)BSA], or saline. Macrophage infiltration was measured by immunohistochemical staining, apoptotic cells were detected by in situ end labeling, and proliferating cells were identified by in situ hybridization for histone mRNA. Macrophage infiltration was significantly greater in the FA(+)BSA group than in the FA(−)BSA and saline groups. The infiltrate was largely restricted to the outer cortex. Apoptosis was greater in the FA(+)BSA group than in the FA(−)BSA and saline groups. Compared with the saline group, apoptosis was significantly increased in the FA(+)BSA group but not in the FA(−)BSA group. Cortical cells proliferated significantly more in the FA(+)BSA and FA(−)BSA groups than in the saline group. FA(+)BSA is therefore a more potent inducer of macrophage infiltration and cell death than FA(−)BSA. The fatty acids carried on albumin may be the chief instigators of tubulointerstitial injury in protein-overload proteinuria.