Polyamine concentrations, ornithine decarboxylase (ODC) activity and S-adenosyl-L-methionine decarboxylase activity in the spleen of ICR mice after s.c. inoculation of 2 X 10(6) Sarcoma-180 ascites tumor cells were studied. ODC activity and putrescine concentration in spleen were markedly elevated at 1 and 5 d after inoculation of tumor cells, and spleen weight increased from 3 to 7 d after the inoculation. The elevations at 5 d after inoculation corresponded to the enlargement of spleen. By s.c. inoculation of 2 X 10(6) METH-A ascites tumor cells into BALB/c mice, ODC activity and putrescine concentration in the spleen were also elevated at about 1 d after inoculation. On the other hand, when normal cells (spleen cells) were inoculated into ICR mice, those variations were not observed. From these facts, it was thought that elevations of ODC activity and putrescine concentration in the spleen at 1 d after inoculation of tumor cells were related to the immune response. Further, those were elevated according to the number of inoculated cells, and when cell membranes of 2 X 10(6) Sarcoma-180 ascites tumor cells were inoculated, those were also elevated at about 1 d after the inoculation. From the above facts, it was proved that elevations of ODC activity and putrescine concentration in the spleen at 1 d after the inoculation of tumor cells were due to antigenicity of tumor cell membranes and those were dependent on the number of inoculated tumor cells.