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I. Exogenous l-palmitoyl-sn-glycero-3-phosphorylcholine becomes incorporated into the membrane of A. castellanii within 2 min of incubation. 2. Homogenates of A. castellanii are shown to contain phospholipase A activity. 3. The phospholipase A activity is dependent on the population density of the culture. 4. An increased phospholipase A activity in older cultures of A. castellanii, due to an increased population density, induces aggregation of these cells in a way that communicative contacts are formed. 5. At pH 5.5, the pH value at which the cells were grown, the phospholipase A activity is stimulated by CAMP; at pH 8.0 no stimulation occurs.

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