The effects of Lysozyme (hen egg-white lysozyme) and of its modified derivative mPEG-Lyso, (Lysozyme coupled with monomethoxypolyethylenglycol) were tested on CBA mice bearing MCa mammary carcinoma. mPEG-Lyso, given by the oral route at a dose comparable to 100 mg/kg/day of native Lysozyme, is at least as active as Lysozyme for the activation of lymphocytes obtained from different districts along the axis GALT-spleen. These effects were evidenced by measuring the in vitro response of lymphocytes of animals treated in vivo with ConA and LPS using the SRB test, and measuring the content of nucleic acids by cytofluorimetric analysis. Lymphocytes obtained from the mesenteric lymph nodes of animals treated with mPEG-Lyso, show a response to ConA and to LPS at early stages of treatment, when tumor growth reduces the response to controls. mPEG-Lyso, was also effective on lung metastasis formation. Considering that mPEG-Lyso,, compared to the native Lysozyme, completely lost its enzymatic action on Micrococcus lysodehycticus cell walls, this data suggest that the effects of lysozyme on immunity and on tumour growth are unrelated to the production of immunoactive peptidoglycans in the gut.