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