A lipoprotein diacylglyceryl transferase (lgt) deletion mutant of Staphylococcus aureus SA113 was constructed. The lipoprotein and prelipoprotein expression, the growth behavior, and the ability of the mutant to elicit an immune response in various host cells were studied. In the wild type, the majority of [14C]palmitate-labeled lipoproteins were located in the membrane fraction, although some lipoproteins were also present on the cell surface and in the culture supernatant. The lgt mutant completely lacked palmitate-labeled lipoproteins and released high amounts of some unmodified prelipoproteins, e.g., the oligopeptide-binding protein OppA, the peptidyl-prolyl cis-trans isomerase PrsA, and the staphylococcal iron transporter SitC, into the culture supernatant. The growth of the lgt mutant was hardly affected in rich medium but was retarded under nutrient limitation. The lgt mutant and its crude lysate induced much fewer proinflammatory cytokines and chemokines in human monocytic (MonoMac6), epithelial (pulmonary A549), and endothelial (human umbilical vein endothelial) cells than the wild type. However, in whole blood samples, the culture supernatant of the lgt mutant was equal or even superior to the wild-type supernatant in tumor necrosis factor alpha induction. Lipoprotein fractionation experiments provided evidence that a small proportion of the mature lipoproteins are released by the S. aureus wild type despite the lipid anchor and are trapped in part by the cell wall, thereby exposing the immune-activating lipid structure on the cell surface. Bacterial lipoproteins appear to be essential for a complete immune stimulation by gram-positive bacteria.