Treatment of C3H mice with partially purified or highly purified virus-induced interferon resulted in a marked increase in the expression of Fcgamma receptors (Fcgamma R) on splenic lymphocytes, mesenteric lymph node cells and thymocytes from cortisone-treated mice. An increase on Fcgamma R on splenic lymphocytes was seen as early as 5 h and lasted at least 72 h after a single injection of interferon. We suggest that enhancement of Fcgamma R is one of the mechanisms by which interferon exerts some immunostimulating effects in vivo.