Human recombinant lysozymes (rHLZs), particularly hen egg-white lysozyme (HEL), are promising agents for the treatment of diseases such as cancer. However, in a similar but improved fashion to what has already been demonstrated using HEL, the PEGylation of an rHLZ leads to a new drug that appears to protect against spontaneous lung metastasis development in mice bearing mammary carcinoma (MCa). The oral administration of 25-100 mg/kg/day of rHLZ-PEG (HEL-equivalent dose) to CBA female mice, admixed with daily food for 14 consecutive days, significantly reduced the growth of the primary tumour by up to 30% and of lung metastasis weight by up to 95%, as compared to the untreated controls. An anti-metastatic effect significantly higher than that of uncoupled rHLZ was also confirmed in MCa-carrying animals immunosuppressed by cyclophosphamide or cyclosporine. This is of note if one takes into consideration the fact that virtually all chemotherapeutic regimens can cause immune system depression, with a consequent limitation of the dosage and effectiveness of anti-tumour treatments.