Human lymphotoxin was purified to homogeneity from a serum-free tissue culture supernatant of a lymphoblastoid 1788 cell line. The purification scheme consisted of DEAE-cellulose chromatography, preparative isoelectric focusing, lentil lectin-Sepharose chromatography, and preparative polyacrylamide gel electrophoresis. The purified glycoprotein was homogeneous by the criteria of high pressure liquid chromatography and polyacrylamide gel electrophoresis run under both nondenaturing and denaturing conditions. The specific activity of the purified lymphotoxin is approximately 40 X 10(6) units/mg. The protein has an apparent molecular weight of approximately 20,000, and RF of 0.33 on 7.5% polyacrylamide gels at pH 8.8 and an isoelectric point of 5.8. A tryptic digest of the purified native material produced two major fragments of approximately 15,000 and 5,000 Da. The amino acid compositions of the intact molecule and of the tryptic fragments are presented.