Large granular lymphocytes obtained from murine blood (B-LGL) and intestinal epithelium (IE-LGL) are cells associated with natural killer (NK) activity and thought to be a first line of defense against tumors and/or infectious organisms. Since B-LGL and IE-LGL represent circulating and mucosal NK effectors, respectively, we compared their surface markers, NK activity and morphology to define possible differences between NK cells in different anatomical compartments. B-LGL and IE-LGL were purified by Percoll gradient centrifugation from nude, normal, and beige C57BL/6 mice. We have defined the following surface phenotypes. B-LGL: In nude mice most of them expressed T-200 (89%), asialo-GM1 (71%), and NK-1.1 (72%); 15% possessed the Thy-1.2 antigen, few cells expressed Ly-2, and none showed Ly-1 positivity. Beige mouse B-LGL were positive for T-200 and NK-1.1. IE-LGL; Nude IE-LGL compared to nude B-LGL showed a similar expression of T-200 and Thy-1.2. Ly-1+ and Ly-2+ cells were more numerous than in B-LGL, whereas NK-1.1+ and asialo-GM1+ cells were less numerous. Interestingly, Ly-2+ IE-LGL were at least partially Thy-1.2-. In euthymic mice IE-LGL had a phenotype comparable to that of nude IE-LGL. The NK activity of B-LGL from nude and normal mice was considerably higher than that of IE-LGL from the corresponding mice. IE-LGL from nude mice possessed larger cytoplasms, and more numerous and bigger azurophilic granules than B-LGL. Similar findings were obtained in normal mice. In beige mice 95% of B-LGL showed a single granule whereas 80% of IE-LGL contained multiple granules (mean 3/cell). Giant granules were frequently found in beige IE-LGL while they were rare in beige B-LGL. Thus, clear differences exist between B-LGL and IE-LGL and they may reflect either different homing patterns of subpopulations of LGL or different stages of maturation of the same lineage of cells.