The serum factor responsible for the humoral leukocyte adherence inhibition (H-LAI) reaction in lung cancer patients has been purified. It is precipitable by ammonium sulfate between 30-70% saturation. On DEAE ion exchange chromatography activity is eluted in the 0.12-0.2 M acetate fraction. The serum factor has affinity for Con A. This gives evidence for a glycoprotein nature of the factor. Electrophoresis indicates an apparent mol. wt of 71,000 dalton. The data suggest that the protein can be separated into subunits of 43,000 and 28,000 dalton under reducing conditions. Isoelectric focusing gives a mean pI of 4.5. The applied fractionation procedure gave a 1120-fold purification relative to lung cancer serum. Antiserum against the purified factor has been produced in rabbits. The factor can be demonstrated in high concentrations in serum of lung cancer patients, but is also found in smaller quantities in sera of other types of cancer. Minor quantities are present in normal serum. The results suggest that quantitation of the H-LAI factor can be used in cancer diagnosis.