Objective: Urolithiasis and free radicals have long been associated. In this study, we have isolated calcium oxalate monohydrate (COM) binding proteins from rat kidney before and after lipid peroxidation (LPO) and studied its properties on calcium oxalate crystal growth. Materials and Methods: LPO was carried out using t-butyl hydroperoxide, cumene hydroperoxide and an ascorbate system. The COM binding proteins from control and peroxidised tissues were isolated using a modified procedure. Protein was extracted using 25 m M EDTA, and the extract was loaded onto a DEAE cellulose column and eluted with Tris-HCl buffer (pH 6.5), 0.05 M NaCl in the above buffer and 0.3 M NaCl in the same buffer. Three major protein fractions were obtained, and they were designated as fractions I, II and III according to their order of elution. The proteins were subjected to calcium oxalate crystal nucleation and aggregation. Results: A positive correlation was observed between LPO and COM adsorption, while a negative correlation was observed between reduced glutathione and COM adsorption. Peroxidised protein did not show any alteration in the elution profile on the DEAE cellulose column. The –SH content of the peroxidised fractions were lower than that of the control fractions, but their oxalate binding activities were increased. Peroxidised fraction I promoted crystal growth to a greater extent than the control fraction I. Peroxidised fractions II and III were less inhibitory in nature compared to their control fractions. Light-microscopic examination of the crystals formed in the presence of the peroxidised fractions showed the formation of large aggregates of COM. Conclusion: Peroxidation of the renal proteins favoured their adsorption to COM crystals. –SH depletion increased the oxalate binding activity and also their affinity to the COM crystals. The peroxidised fraction I was found to favour the formation of large aggregates, suggesting that peroxidation may be one of the mechanisms altering the crystal inhibitory activity of the proteins in hyperoxaluria.