The aim of the present investigation was to examine the effects of the lysozyme derivative mPEG-lyso (hen egg-white lysozyme coupled with polyoxyethylenglycol), on TS/A adenocarcinoma cell line in vivo and in vitro. mPEG-lyso reduces the number of ICAM-1+ and E-cadherin+ cells of TS/A adenocarcinoma cell line in vitro, and causes a marked decrease of spontaneous lung metastases in vivo. In both cases, mPEG-lyso reduces the number of tumour cells in sythesis and pre-mitotic phases. In connection with the reduction of cells expressing adhesion molecules, mPEG-lyso reduces the number of infiltrating leukocytes in the primary tumour in vivo and reduces the binding capacity of splenocytes to tumour cells in vitro. These data stress, for the first time, that the in vivo control of mPEG-lyso on lung metastasis formation of solid metastasising tumours may be due to a combination of effects on tumour cells in addition to those on host's immune system.