Active acid phosphatase (AcP), 37 kDa, was completely separated from the purified lactoferrin fraction of bovine milk (LF-rich fraction). The N-terminal amino acid sequence of the 37 kDa molecule had 84% homology with bovine uteroferrin (Uf)-like protein. The 37 kDa molecule has an optimum pH range of 4.5-5.0 and an optimum temperature of 60 degrees C. The AcP activity of the iron removal 37 kDa molecule (iron-depleted LF-rich fraction) was approximately half that of the iron-containing sample (LF-rich fraction). The activity increased in a time-dependent manner on tryptic digestion. These profiles correspond to the mammalian purple acid phosphatase (PAP) family (Uf, Type-5 and tartrate-resistant acid phosphatase: EC3.1.3.2). It seems reasonable to propose that the active molecule in the LF-rich fraction is an undocumented bovine PAP-family protein. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.