Lactoferrin (LF) is reported to stimulate osteoblast proliferation and inhibit osteoclast activity in bone cell culture. However, the effect of oral LF on bone in osteoporosis needs to be explored. Three-month-old female Sprague-Dawley rats (n = 70) were assigned to the following groups: sham-operated, ovariectomized (OVX) untreated, OVX + bovine serum albumin (BSA; 85 mg/kg body weight), OVX + LF (0.85 mg/kg, 8.5 mg/kg, and 85 mg/kg body weight), and OVX + 17beta-estradiol (E(2); 10 microg/kg body weight). After 3 mo of treatment, E(2) completely prevented the OVX-induced bone loss. OVX rats treated with LF were protected against the OVX-induced reduction of bone volume, trabecular number, and thickness, and the elevation of trabecular separation was prevented. LF also increased bone mineral density and increased the parameters of mechanical strength at 8.5- and 85-mg/kg doses. Greater bone formation and reduced bone resorption, as assessed by biochemical markers of bone remodeling, occurred in rats administered LF. LF at 8.5- and 85-mg/kg concentrations caused a significant decrease in serum calcium, but this reduction did not occur in rats fed 0.85 mg/kg LF. In addition, serum tumor necrosis factor-alpha and interleukin-6 production were suppressed and serum calcitonin was elevated significantly in LF-fed rats at all 3 doses. These findings indicated that oral LF not only preserved bone mass but also improved bone microarchitecture. The absorption of LF peptides and their effects on bone cells could to some extent account for the osteogenic function of oral LF.