A highly purified, non-hemagglutinating PHA used in minimal blastogenic doses was non-toxic to established human lymphoid cell lines. PHA given at the beginning of primary culture or repeatedly throughout the initial six weeks of culture did not interfere with the establishment of peripheral lymphocytes in long-term suspension culture. Permanent cell lines have been obtained following weekly administration of PHA in blastogenic and subthreshold doses. Nonhemagglutinating PHA appeared to be non-toxic to these peripheral lymphocyte cultures and may increase their potential for long-term proliferation.