Increasing blood CO concentrations in C26 solid tumor-bearing mice accompanied with tumor growth and increased HO activity in tumor. Murine colon cancer C26 solid tumor was obtained by injecting cultured C26 cells (2 × 106) subcutaneously in the dorsal skin of Balb/c mice. After scheduled times, CO concentrations in circulation was detected by gas chromatography (a), and tumor volume was measured (b). Inset of (a) shows the HO activity in tumors at different times after tumor inoculation. Data indicate that the blood CO concentrations increase in parallel with tumor growth. See text for details.