Natural killer (NK) cell functional activity is absent in mice < 3 weeks of age. However, the mechanism(s) responsible for such inactivity is (are) still unknown. Some evidence suggests basic immaturity of the infant-source NK cells while others suggest suppression of NK cell-mediated activity by other endogenous cells/factors. In the present study, infant-source, spleen-derived, NK cells were highly enriched using anti-NK 1.1 monoclonal antibody coupled to immunomagnetic DYNAbeads. The NK cell characteristics of the isolated cells were tested using three assays: (i) target-binding capacity, (ii) lytic potency as measured in a single cell assay using propidium iodide to label NK cell-lysed dead targets and (iii) proliferative capacity (3H-thymidine incorporation). The results demonstrated that enriched populations of infant NK cells have a greater capacity to bind target cells than unseparated infant or young adult spleen cells. Nevertheless, both unseparated spleen cells and isolated NK cells had a similar 3H-thymidine uptake. The NK cell-mediated lytic capacity of unseparated infant spleen cells was negligible, with 0.5% of YAC-1 targets being killed. However, the lytic capacity of isolated infant-source NK cells was 12% and could be further augmented to 21% after exposure to recombinant interleukin-2. These results suggest that (a) NK cells are present and functional in infant mice, but are suppressed by some endogenous cells/factors, (b) such suppression acts at a postbinding event of NK lysis and (c) such suppression is released when NK cells in infant spleen are removed from that environment.