To investigate the effect of hyperosmotic medium on production and aggregation of the variant of Angiopoietin-1 (Ang1), cartilage oligomeric matrix protein (COMP)-Ang1, in recombinant Chinese hamster ovary (CHO) cells, CHO cells were cultivated in shaking flasks. NaCl and/or sorbitol were used to raise medium osmolality in the range of 300-450mOsm/kg. The specific productivity of COMP-Ang1, q(COMP-Ang1), increased as medium osmolality increased. At NaCl-450mOsm/kg, the q(COMP-Ang1) was 7.7-fold higher than that at NaCl-300mOsm/kg, while, at sorbitol-450mOsm/kg, it was 2.9-fold higher than that at sorbitol-300mOsm/kg. This can be attributed to the increased relative mRNA level of COMP-Ang1 at NaCl-450mOsm/kg which was approximately 2.4-fold higher than that at sorbitol-450mOsm/kg. Western blot analysis showed that COMP-Ang1 aggregates started to occur in the late-exponential phase of cell growth. When sorbitol was used to raise the medium osmolality, a severe aggregation of COMP-Ang1 was observed. On the other hand, when NaCl was used, the aggregation of COMP-Ang1 was drastically reduced at NaCl-400mOsm/kg. At NaCl-450mOsm/kg, the aggregation of COMP-Ang1 was hardly observed. This suggests that environmental conditions are critical for the aggregation of COMP-Ang1. Taken together, the use of NaCl-induced hyperosmotic medium to cell culture process turns out to be an efficient strategy for enhancing COMP-Ang1 production and reducing COMP-Ang1 aggregation.