Rabbit antisera against hamster embryo cells and hamster cells which had been transformed by polyoma virus were assayed by single cell technics. Antiserum killed cells in the presence or absence of complement, and the two mechanisms of inactivation appear to be different. Kinetics of cell-killing by antibody and complement are described. The transformed cells studied appear to have less surface antigen than embryo cells. Specific antibody to polyoma virus-transformed hamster embryo cells could not be found in an antiserum prepared against one such polyoma-transformed hamster cell line. The methodology is reported since it has proved capable of resolving antigenic differences between mammalian cells in the sense that an unabsorbed antiserum having equally strong sensitizing effects on two different cell types could be shown to specifically sensitize only one cell type after exhaustive absorption with the other.

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