Summary: A lymphogranuloma venereum complement-fixing antigen was extracted from infected yolk sac suspensions with anesthetic ether. The activity of the ether extract antigen represented four- to eight-fold enhancement. Maximal enhancement in the ether extract antigen depended on (a) preliminary aging of the yolk sac suspensions, (b) degree of dispersion of the extract in saline solution, and (c) absence of phenol. The purified ether extract antigen possessed the same complement-fixing group reactivity as phenolized unextracted antigen. The similarity of extract antigen to glucolipid fractions of bacterial origin was discussed.