Human lymphocytes sensitized to allogeneic determinants in vitro were continuously cultured on medium prepared from phytohemagglutinin- (PHA-P) stimulated human mixed lymphocyte cultures. With such human-conditioned medium (HCM), human peripheral lymphoid cells could be grown in culture for over 90 days with a doubling time of about 54 hr. Cytotoxic lymphocytes could be grown in culture for this time with no loss of cytotoxicity or change in cytotoxic specificity.