The antiproliferative activities of human lymphoblastoid interferon-alpha (MOR-22) against 22 cultured human cell lines were examined. Daudi cells and ACHN cells were highly sensitive to interferon compared with other human cells. The cell lines derived from renal and brain tumors showed concentration-dependent growth inhibition as determined from a 7-day growth curve but the concentration dependency was different in various cell lines. It was shown that longer exposure to interferon was necessary to obtain antiproliferative activity, and cell growth inhibition was completely reversible by washing the cultures. It was suggested that interferon had a time-dependent antiproliferative action.