The intercellular adhesiveness of density-inhibited (D.I.) and fast-growing (F.G.) HeLa cells and of trypsin-treated preparations of these, has been measured at temperatures between 37 and 6 degrees C. In EDTA-containing buffer medium, F.G. cells differ from D.I. cells in that only the former display an increase in adhesiveness below 30 degrees C. This increase is prevented by previous treatment with trypsin. The presence of Ca2+ in the buffer medium causes a narrowing of the thermal transition region of intact F.G. cells. On intact D.I. cells Ca2+ causes an increase in adhesiveness at temperatures below 20 degrees C. Previous trypsinization of F.G. cells diminishes the effect of subsequent Ca2+ addition. The adhesiveness of trypsinized D.I. cells is indifferent to changes in temperature in Ca2+-containing buffer medium. The results are considered evidence for the occurrence of a phase transition in the glycoprotein domains of the plasma membrane of fast-growing cells. The transition is influenced by growth rate, trypsinization and Ca2+. The fluidity of the membrane glycoproteins is considered to be higher on density-inhibited cells than on fast-growing cells. No phase transition could be detected after incorporation of the fluorescent compound 1,6-diphenyl 1,3,5-hexatriene into the lipid domain of the plasma membrane. The fluidity of the membrane lipids is lower on density-inhibited cells than on fast-growing cells.

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