Abstract
The present study suggests that pretreatment of mice with cyclophosphamide (CPA) not only suppresses the intestinal immune response to malignant ascites cells but alters the character of the response. Non-treated animals were sensitized by intestinal immunization while CPA-treated animals showed enhanced tumour growth. Sera from enhanced mice contained factors which at the tumor cell membrane were able to interfere with the binding of antibodies detectable by immunofluorescence technique. These factors might be immunoglobins with similarities to IgG, but bound to antigen they were undetectable by the fluorescence method. Enhanced mice were found to be immunologically hyporesponsive to the challenging tumor graft as evidenced by a delay in the immune response to the graft.