AbstractComplement‐fixing (CF) antigen, prepared from cytomegalovirus (CMV)‐ infected cells and solubilized by urea and Tween‐20, was fractionated on agarose gel columns. Its molecular weight was 1.5−5 × 107daltons, and it contained 5 major peptides with molecular weights from 60,000–90,000 daltons. The peptide with a molecular weight of 70,000 daltons which was present in the solubilized CF antigen was absent in a soluble fraction from infected cells without CF activity and may be, therefore, important in this respect. The antigens present in the CF antigen did not elicit neutralizin