1. The ability of a number of carboxylic acids, their esters, retinol and α-tocopherol to induce fusion of hen erythrocytes in vitro was investigated. 2. Some 30 different fat-soluble substances (100μg/ml) were found to cause the formation of multinucleated erythrocytes with a suspension of 3×10 8 erythrocytes/ml. The most effective agents induced fusion within 5–10min at 37°C; some substances required about 1h. 3. Inclusion of Dextran 60C in the test medium minimized colloid osmotic lysis caused by exogenous lipids that induce cell fusion. 4. Cell swelling, followed by cell adhesion, was then seen to precede cell fusion. 5. Fusion occurred with C 10 –C 14 saturated carboxylic acids, with unsaturated, longer-chain carboxylic acids and their mono-esters; retinol, and to a lesser extent α-tocopherol, also caused cell fusion. 6. C 6 –C 9 , C 15 , C 16 and C 18 saturated carboxylic acids did not induce fusion within 4h; glyceryl dioleate was only weakly active, and glyceryl trioleate was inactive in the test system. 7. Fusion was facilitated by a high ratio of chemical agents to cell number and by incubation between pH5 and 6. It was inhibited by EDTA and by serum albumin. 8. Glyceryl mono-oleate caused both a similar fusion of several species of mammalian erythrocyte and the interspecific fusion of human and chicken erythrocytes. 9. The term ‘fusogenic’ is proposed to describe chemical, viral and physical agents that cause membranes to fuse. 10. The biochemical mechanisms involved and the possible biological significance of membrane fusion by fusogenic lipids are discussed.

1. A simple two-phase chloroform–aqueous buffer system was used to investigate the interaction of insulin with phospholipids and other amphipathic substances. 2. The distribution of 125 I-labelled insulin in this system was determined after incubation at 37°C. Phosphatidic acid, dicetylphosphoric acid and, to a lesser extent, phosphatidylcholine and cetyltrimethylammonium bromide solubilized 125 I-labelled insulin in the chloroform phase, indicating the formation of chloroform-soluble insulin–phospholipid or insulin–amphipath complexes. Phosphatidylethanolamine, sphingomyelin, cholesterol, stearylamine and Triton X-100 were without effect. 3. Formation of insulin–phospholipid complex was confirmed by paper chromatography. 4. The two-phase system was adapted to act as a simple functional system with which to investigate possible effects of insulin on the structural and functional properties of phospholipid micelles in chloroform, by using the distribution of [ 14 C]glucose between the two phases as a monitor of phospholipid–insulin interactions. The ability of phospholipids to solubilize [ 14 C]glucose in chloroform increased in the order phosphatidylcholine<sphingomyelin<phosphatidylethanolamine<phosphatidic acid. Insulin decreased the [ 14 C]glucose solubilized by phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid, but not by sphingomyelin. 5. The significance of these results and the molecular requirements for the formation of insulin–phospholipid complexes in chloroform are discussed.