This study establishes that natural killer (NK) cells cytolytic for B-lymphoblastoid cell lines (B-LCL) expressing NK-defined alloantigens can be stimulated to proliferate in culture independently of allogeneic stimulation. NK cells proliferate following co-culture with a gamma-irradiated malignant melanoma cell line (MM-170) and IL-2-conditioned medium. The cultured NK cells from some donors showed a high level of cytotoxicity against NK-1+ B-LCL and this corresponded with a high precursor frequency (46-64%) determined from limiting dilution analysis. Alloreactive NK cells proliferated in cultures containing autologous activated T cells, demonstrating that alloantigens were not essential to stimulate proliferation. B-LCL expressing NK-1 or NK-2 or neither of these alloantigens stimulated proliferation of NK cells cytolytic for NK-1+ B-LCL. Studies using metabolically inactivated B-LCL confirmed that stimulation was not alloantigen dependent. The results demonstrate that recognition of alloantigens by NK cells, sufficient to trigger their lytic program, is not required and indeed is not sufficient to confer a stimulatory signal for proliferation of alloreactive NK cells.