Abstract We examined the response of cells of murine gut-associated lymphoid tissues to agents that augment the activity of natural killer (NK) cells. Specifically, we studied the effect of polyinosinic: polycytidylic acid (Poly I:C) in vivo, and recombinant interferon α A D (rIFNα A D ) and recombinant interleukin 2 (rIL2) in vitro on lymphoid cells of mesenteric lymph nodes (MLN) and Payer's patches (PP) in generating cytotoxicity against NK-sensitive (YAC-1) and NK-insensitive (B16BL6) tumor targets. The effect of these agents on spleen cells was examined for comparison with their effect on MLN and PP cells and as a positive control. MLN and PP cells lacked spontaneous NK activity; however, NK activity could be augmented to different levels by the three agents. The treatment of mice in vivo with Poly I:C induced considerable cytotoxicity in the spleen and MLN but only a weak cytotoxic response in PP. The in vitro enhancement of NK activity by rIFNα A D was strong in the spleen, intermediate in MLN, and consistently poor in PP. The weak NK augmentation by rIFNα A D in PP was not restricted to a single mouse strain. PP cells from five strains of mice responded poorly to rIFNα A D . Furthermore, NK augmentation by rIFNα A D in PP cells did not improve after passing the responder cells through nylon wool, indicating that the lack of augmentation of NK activity was not the result of a preponderance of B cells or the masking of NK cells by adherent lymphoid populations in PP. In contrast to weak augmentation of NK activity by rIFNα A D , considerable IL2-induced lymphocyte-activated killer (LAK) activity against NK-insensitive B16BL6 tumor cells was induced in PP. Limitingdilution analysis showed that the frequency of LAK precursors in the MLN and PP was not markedly different from that of the spleen. The differences among spleen, MLN, and PP lymphoid populations in generating the broadly nonspecific cytotoxic effector cells in response to rIFNα A D or rIL2 may result from differences in the pools of different pre-NK cells or to differential sensitivity of the same pool of pre-NK cells to rIFNα A D and rIL2 in different anatomical locations.