1. The N-(2-hydroxyethyl)alanine esterified to phosphatidic acid in anaerobic ciliate rumen protozoa has the l configuration. 2. Labelling experiments with Entodinium caudatum cultures using [32P]Pi [2-14C]ethanolamine and 32P- and 14C-labelled phosphatidylethanolamine show that phosphatidylethanolamine is the direct lipid precursor of the N-(2-hydroxyethyl)alanine-containing phospholipid. 3. Labelling experiments with [14C]starch, [14C]lactate and [14C]pyruvate with E. caudatum cultures indicate that a three-carbon glycolytic intermediate is probably the precursor of the N-(1-carboxyethyl) grouping which substitutes on the amino group of phosphatidylethanolamine. 4. [32P]phosphatidylethanolamine is catabolized by E. caudatum forming initially glycerylphosphorylethanolamine and subsequently glycerophosphate and Pi. A little phosphorylethanolamine formed may possibly arise from bacterial enzymes ingested by the protozoa.

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