Abstract
ME1 C9, a cloned line of mouse embryo cells, transformed by SV40 in vitro, and its more malignant progeny, ME1 C9‐V15, derived from a tumor of the 15th consecutive passage of ME1 C9 in vivo, were compared as regards immunogenicity and immunosensitivity in syngeneic CBA mice. A threshold zone of immunization with ME1 C9 and ME1 C9‐V15 cells in the range of 10^3^‐10^5^ irradiated cells was indicated, after which the immunized animals were 1,000 times more resistant. There was no significant difference in immunizing potential between ME1 C9 and ME1 C9‐V15 cells at any dose level. A heterologous line of SV40‐transformed rat cells was significantly less immunogenic than the syngeneic cells. The growth of ME1 C9‐V15 cells could be enhanced after immunization with spontaneously transformed ME3 cells or with a small dose of ME1 C9 cells. The immunosensitivity of the ME1 C9 and ME1 C9‐V15 cells was measured as a sensitivity index in uniformly immunized animals. Both lines appeared immunosensitive if the animals were sufficiently immunized. The less malignant cells, ME1 C9, appeared more immunosensitive than the more malignant ME1 C9‐V15 cells in seven out of nine experiments, but only once was the difference between the lines significant. The interpretation of differences of sensitivity indices is discussed, and a possible role of blocking factors is considered.