Human natural killer (NK) cells play an important role in first-line defence against primary and metastatic tumours. Their stimulation after autologous bone marrow transplantation (ABMT) may be useful to eradicate residual tumour cells. Human NK cells originate in bone marrow (BM) but little is known about their progenitors and lineage development. We studied NK cell ontogeny from BM progenitors obtained by 'purging' normal BM with 4-hydroperoxycyclophosphamide (4HC); this agent is known to destroy all but the most primitive BM progenitors. NK cells, defined by their phenotypic markers and cytolytic activity, could be generated from 4HC-treated BM cells during in vitro cultures over stromal BM feeder layers and in suspension cultures containing a mixture of soluble cytokines; Interleukine 2 appears to be essential for the full development of this population. A lag period exists before NK cells can be found in significant numbers in culture; this suggests that a delay in initiation of immunotherapy after ABMT ought to be considered, particularly when using purged marrow.