Several acute phase reactants have been shown by us and by others to reach abnormally high levels in cancer patients, the more so when the disease is disseminated. These glycoproteins have been tested in vitro for their ability to interfere with PHA blastogenesis and monocyte chemotactism of mononucleated cells from normal donors. It is shown that, used at the concentration they reach in cancer patients, haptoglobin fibrinogen, alpha 2-macroglobulin and transferrin inhibit both tests, whereas orosomucoid inhibits only the in vitro chemotactism of monocytes. It is proposed that these glycoproteins, synthesized by the liver in a response to inflammatory stimuli, are protecting the tumors against immune mechanisms of the host and are therefore part of the immune escape mechanism employed by the tumors.