The number of cells producing the anti-Ehrlich tumor antibody (antibody against Ehrlich tumor cells) in the spleen of mice bearing Ehrlich solid tumor was counted by the modified Cunningham method, in which the sheep red blood cells were conjugated with extracted specific Ehrlich tumor antigen and used. The antibody-producing cells appeared 6 days after the subcutaneous transplantation of Ehrlich tumor cells and increased in number with a peak around the 15th day. Humoral anti-Ehrlich tumor antibody estimated by immune adherence gradually increased in the tumor-bearing mice, reached the maximum 15 days after the transplantation or later, and maintained the high level for a long period. The subcutaneous solid tumor grew and reached the maximum in weight around the 15th day and regressed thereafter.