AbstractNatural killer (NK) cells are likely involved in the control of early phase of the intracellular bacterial Listeria Monocytogenes (LM) -infection, as the athymic nude rats, which have increased NK activity and minimal T-cell activity, are more resistant to early bacterial growth than wild type rats. The expression of Ly49 receptors vary between different rat strains. We investigated PVG-, AO- and DA-rats and found no correlation between growth of LM in vivo and expression of Ly49. Further, NK cell depletion in vivo with the mAb anti-NKR-P1A/B, lead to an increased splenic bacterial load. In rats infected with LM the splenic Ly49+ subset increased significantly, while the Ly49- subset decreased.