A certain number of CBA mice injected intraperitoneally with 1 × 106 or fewer Ehrlich ascites tumor (EAT) cells did not develop ascites tumors but solid tumors at the inoculation site. The incidence of solid tumors proved dependent on the level of immunological reactivity of recipients, being increased in mice with increased immunological potency and absent in mice in which this potency was reduced. Mice bearing solid tumors had increased level of cytotoxic antitumor antibodies in serum. Possible reasons for a greater immune resistance of cells growing in subcutaneous tissue, than in the abdominal cavity, are discussed