1. Choline- and inositol-labelled phospholipids of exponentially growing or static neoplastic mast cells turn over by degradation and resynthesis of the entire molecule. Turnover follows a biphasic pattern, the unstable rapidly turning-over component accounting for 60–80% of labelled phospholipid. The residual stable component does not turn over any more than does protein or DNA. 2. Subcellular fractions and surface membranes of choline-labelled P815Y cells contain the same proportion of stable and unstable components as do whole cells. The unstable component is largely phosphatidylcholine; the stable component is relatively richer in sphingomyelin. 3. It is concluded that the phospholipids of neoplastic mast cells are of two classes, one of which is susceptible to continual enzymic degradation and resynthesis, and the other of which is metabolically stable.

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