Paramecium caudatum has been shown to cause the flocculation of Indian ink particles. Flocculation has been demonstrated to be due to: (a) the secretion of a substance "P," a carbohydrate whose mono-saccharide constituents are glucose and arabinose, (b) the ingestion of particles by the cell which are then bound together by a sticky mucoprotein. Substance "P" is adsorbed on to Indian ink particles present, which results in a change of their surface charge and the aggregation into floccules. Substance "P" has been labelled with [14C] glucose which then appeared in the floccules.