The growth of 3T3 and SV101 3T3 cells in a lipid-depleted medium is enhanced by the addition of biotin or some fatty acids. The extent of enhancement depends on the fatty acid(s) supplied. The presence of linoleate is unique, since it induces a morphological alteration in 3T3 cells resulting in a cell similar to an SV101-transformed 3T3 cell. Analyses of the fatty acids from the membrane phosphatides show that the exogenously supplied fatty acids are incorporated and alter the fatty acid composition. This is most clearly evident with heptadecanoate-grown cells, in which this fatty acid and its derivatives comprise over 45% of the fatty acids in the phospholipids. The fatty acid replacements have a striking effect on the temperature dependence of agglutination by wheat germ agglutinin and concanavalin A, implying that fluidity is involved in agglutination. These temperature dependencies and the effect of fatty acid replacements on them were different for the two lectins, but similar for both transformed and untransformed cells. These observations are interpreted as suggesting that the lipid phase is heterogeneous, and that transformed and untransformed cell membranes have regions of similar fluidity.