SummaryPhosphatidyl ethanolamine and phosphatidyl choline are inactive in the Thromboplastin Generation Test, but they become highly active when present with about 10% phosphatidyl serine. Phospholipids from fresh eggs are inactive, because they contain no phosphatidyl serine. Sodium deoxycholate gives clear, but less active or inhibitory suspensions.The author is very grateful to Dr. Erich Baer, Toronto, Canada, for a generous gift of synthetic L-α-(Dioleoyl)-phosphatidyl ethanolamine, and thanks Mrs. Jane Harvey and Joann Powers for expert technical assistance.