It has been found out that tripeptide Arg-Gly-Asp being under natural conditions a fibronectin fragment, responsible for adhesion, is capable in vitro to stimulate ingestion ability of the rat blood polymorphonuclear leucocytes and monocytes. Maximum increase of phagocytes index, phagocytes number and phagocyte per cent, which has ingested more than 10 latex particles, has been discovered after leuko-suspension incubation with tripeptide in initial concentrations 10(-3) mM and 10(-2) mM. Tripeptide can be regarded as a perspective phagocyte function regulator.