Angiopoietin-1 (Ang1) is an essential molecule for blood vessel formation. In an effort to produce large quantities of Ang1, recombinant Chinese hamster ovary (rCHO) cells expressing a high level of recombinant human Ang1 protein (rhAng1) with an amino terminal FLAG-tag were constructed by transfecting the expression vector into dihydrofolate reductase-deficient CHO cells and subsequent gene amplification in a medium containing step-wise increments of methotrexate, such as 0.02, 0.08, and 0.32 microM. The rhAng1 secreted from rCHO cells was purified at a purification yield of 18.4% from the cultured medium using an anti-FLAG M2 agarose affinity gel. SDS-PAGE and Western blot analyses showed that rCHO cells secret rhAng1 as heterogeneous multimers. Moreover, rhAng1 expressed in rCHO cells is biologically active in vitro as demonstrated by its ability to bind to the Tie2 receptor and to phosphorylate Tie2. Therefore, the rhAng1 produced from CHO cells could be useful for clarifying the biological effects of exogenous rhAng1 in the future.