The properties of 121-6 cells were characterized. This cell line is transformed by ts-121, a temperature-sensitive mutant of polyoma virus. Both the morphology and saturation density of the 121-6 cells were affected by temperature; i.e. at 39 degrees C, the cells grew to monolayer sheets and remained contact inhibitied for a long time (at least 25 days), while at 35 degrees C, they grew beyond the monolayer density, like cells transformed by wild-type polyoma virus. These phenotypic changes were reversible, but increased agglutinability by Concanavalin A, which was another phenotype of these transformed cells, was not temperature-sensitive, and even at high temperature, 121-6 cells, like transformed cells, showed high agglutinability.