Pregnancy Associated Mouse Protein 1 (PAMP1) was isolated from plasma of female mice. An antiserum raised against the purified protein confirmed its immunochemical identity with the originally described PAMP1. Pregnant females were observed to have plasma levels of PAMP1 that are increased two-fold at day 10-13 of gestation relative to non-pregnant females, while male mouse plasma did not contain PAMP1. The purified protein displayed an apparent subunit molecular mass of 70 kDa, irrespective of cystine reduction. The native molecular mass, estimated by gel-filtration, was about 140 kDa, indicating that PAMP1 is circulating as a non-covalent homodimer. The amino-terminal sequence of the intact protein and the internal sequences of four cyanogen bromide fragments demonstrated that this protein is not related to any known member of the alpha-2-macroglobulin family nor to any protein in the sequence databases. The physicochemical and the sequence data thus establish this protein as a novel, female-specific protein, but unrelated to the Macroglobulin proteinase inhibitor family.