The surface properties of two cell lines, Cl l-82 and CI M-6, from a cultured mouse mammary tumor cell line, FM3A/B, were studied with phytohemagglutinins and the immune adherence test. The transplantabilities of these cells had greatly decreased either spontaneously or after treatment with 4-nitroquinoline I-oxide. The agglutinability of these non-malignant (low-or non-tumorigenic) cells with agglutinins from various plants was markedly higher than that of the original malignant (tumorigenic) cells. The agglutination was inhibited specifically by certain kinds of sugars. The localization of the receptors of the agglutinin on the cell surface was observed with fluorescent agglutinin. Surface antigens were detected by the immune adherence test with anti-CI 1-82 cells, and their content was found to be higher on nonmalignant than on malignant cells. The .increased content was shown to be responsible for induction of strong immunity against malignant cells.Evidence is accumulating that neoplastic transformation may be a reversible process (Braun, 1969;Sugimura, 1970). There are also recent reports of reversion of malignancy in various tumors occurring spontaneously (Rabinowitz and Sachs, 1968; Paraf et al., 1970;Pierce and Wallace, 1971), by treatment with chemicals (Sanders and Burford, 1968; Shoham et al., 1970;Burger and Noonan, 1970; Silagi and Bruce, 1970; Rothchild and Black, 1970), by fusion with normal cplls (Harris et al., 1969; Murayama-Okabayashi et al., 1971) and by some factors in tissues (Paran et al., 1970; Halpern e f a/., 1970; Ichikawa, 1970; Furusawa et a/., 1971 ; Yoshida, 1971).We reported previously that a cultured line of tumor cells, FM3A/B, originating from a spontaneous mammary adenocarcinoma in a C3H/He mouse yielded cell lines in which tumorigenicity was much reduczd either spontaneously (Hozumi and Nakamura, 1970) or on treatment with 4-nitroquinoline 1-oxide (Koyama and Ishii, 1969). The cell lines with high and low tumorigenicity had the same growth rate, saturation density, efficiency of colony formation in soft agar and chromosome number. Thus, their difference in tumorigenicity was the only observed character distinguishing them.This paper reports studies showing that nonmalignant cells have an increased number of membrane components reacting with phytohemagglutinins and tumor-specific antisera. The localization of the receptors on the cell surface 'was observed using fluorescent agglutinin. The receptor sites on the surface for agglutinins were shown to be some kinds of sugars. The increased number of surface antigens detected by the immune adherence test in cells with reduced tumorigenicity was found to be responsible for induction of strong immunity against malignant cells. MATERIAL AND METHODS CellsThe cells used in this experiment were four clones, FM3A/B (Nakano, 1966) Cells were cultured as described in the previous paper (Hozumi and Nakamura, 1970). MiceSyngeneic C3H/He mice to the original mammary carcinoma cell line, FM3A/B, were used throughout. The mice were kept...