Summary: An active complement-fixing fraction was extracted with anesthetic ether from suspensions of yolk-sacs infected with the viruses of lymphogranuloma venereum, meningopneumonitis and mouse pneumonitis. The acetone and alcohol insolubility of the active substance was utilized in the preparation of a highly purified fraction. The purified fraction, inactive in itself, was activated by the addition of optimal amounts of lecithin of either yolk-sac or vegetable origin. The ether soluble antigen in the form of a saline suspension showed no loss of activity when stored at refrigerator temperature for more than 18 months. Chloroform extracts of infected yolk-sac suspensions were similar to the ether extracts in that most of the activity was in the extract. Benzene extracts showed slight activity and petroleum ether extracts were inactive, whereas the suspensions after such extractions showed markedly enhanced activity. Purified antigens and ether-extracted suspensions were group-reactive in complement-fixation with serums of patients infected with viruses of the psittacosis-lymphogranuloma venereum group. The purified ether-extract antigen to which lecithin was added was a highly satisfactory antigen for use in complement-fixation tests for infection with the viruses of this group.