In order to directly demonstrate the roles of CD8(+) T lymphocytes in non-human primates, in vivo depletion of the CD8(+) T cells by administration of a CD8-specific monoclonal antibody (mAb) is one of the crucial techniques. Recently, the common marmoset (Callithrix jacchus), which is classified as a New World monkey, has been shown useful as an experimental animal model for various human diseases such as multiple sclerosis, Parkinson's disease and a number of infectious diseases. Here we show that an anti-marmoset CD8 mAb 6F10, which we have recently established, efficiently depletes the marmoset CD8+ T lymphocytes in vivo, i.e., the administration of 6F10 induces drastic and specific reduction in the ratio of the CD8(+) T cell subset for at least three weeks or longer. Our finding will help understand the pivotal role of CD8(+) T cells in vivo in the control of human diseases. (C) 2013 Elsevier B.V. All rights reserved.