The Walker carcinosarcoma cells were cultured in the medium with low concentration (0.25%) of fetal calf serum, in the presence of native or methylamine-treated (modified) human alpha 2-macroglobulin (alpha 2-MG) in vitro. The proliferative activity (3H-thymidine incorporation) of these cells increased in the presence of one or the other alpha 2-MG preparation. It allows to suppose that the human alpha 2-MG may serve as a growth-stimulating factor of tumour cells.