By combining velocity and linear density fractionations as well as target cell rosetting techniques we have isolated and morphologically identified the human effector cell type responsible for spontaneous, trypsin-augmentable cytotoxicity against chicken red cells and human myeloma cell line targets. This cell is a large lymphoid cell with strong alpha-naphthyl esterase activity concentrated in a limited area in the cytoplasm usually at the indentation site of a slightly reniform nucleus. Cells with this morphology also formed plaques on chicken erythrocyte monolayers. The cell is nonphagocytic and nonadherent, it carries Fc receptors but no complement receptors on its surface, and shows a weak affinity to sheep red blood cells (SRBC). The frequency of these cells based on morphological analysis is 3 x 10(4)--6 x 10(4)/ml in normal human blood. This cell shows similarities (surface Ig-, Fc+, C3-) with the human natural killer (NK) cells lytic to hematopoetic target cell lines but differs in that the cytotoxicity is augmentable by trypsin and the affinity to SRBC is lower. Therefore, we postulate that these two killer populations represent different subpopulations of human NK cells.