Lactoferrin (LF) is a Fe3+-binding glycoprotein, first recognized in milk and then in other human epithelial secretions and barrier fluids. Many different functions have been attributed to LF, including protection from iron-induced lipid peroxidation, immunomodulation and cell growth regulation, DNA binding, and transcriptional activation. Its physiological role is still unclear, but it has been suggested to be responsible for primary defense against microbial and viral infection. We present evidence that different subfractions of purified human milk LF possess five different enzyme activities: DNase, RNase, ATPase, phosphatase, and malto-oligosaccharide hydrolysis. LF is the predominant source of these activities in human milk. Some of its catalytically active subfractions are cytotoxic and induce apoptosis. The discovery that LF possesses these activities may help to elucidate its many physiological functions, including its protective role against microbial and viral infection.