Summary: A method for preparing an antiserum against guinea pig lymphocytes has been described. The injection of this antiserum into guinea pigs was found to cause a considerable lymphopenia which persisted for 40 hours or longer after a single injection, and which could be prolonged for at least 10 days by repeated injections. In all other ways the leucocytes in the peripheral blood of guinea pigs reacted similarly to injections of a control serum lacking specific antibodies against guinea pig tissues, except that there was a slightly greater neutrophil leucocytosis after antilymphocytic serum than after control serum.