A process for purifying RNAses and growth factors from milk or lactoferrin comprises subjecting the milk or lactoferrin to filtration with a filter of MWCO 30-50 kD to separate it into a retentate fraction comprising lactoferrin and a permeate fraction comprising growth factors and/or RNAses. During filtration with the filter of MWCO 30-50 kD the milk or lactoferrin is subjected to salt treatment such that growth factors and/or RNAses flow into the permeate, the salt treatment comprising maintaining the milk or lactoferrin at a salt concentration of at least 0.2M NaCl or equivalent or a conductivity of at least 20mS/cm during the filtration.
