Immunoperoxidase technique together with electron microscopy shows that lipoteichoic acid (LTA) ofStaphylococcus aureusCowan I is attached to the membrane and penetrates the whole cell wall. A diffuse zone outside and no peroxidase reaction product inside the wall when whole cells were treated with antibody prior to embedding may indicate (i) that LTA is exposed to reaction with antibodies outside the wall, and (ii) that all or most of the anti‐LTA antibodies used are of the IgM class and thus unable to penetrate the wall. Thin sections of strain Wood 46 showed the same picture as Cowan I, but treatment of whole cells before embedding gave no diffuse zone outside the wall. This may be due to a thicker wall as found by electron microscopy and/or shorter LTA‐chains of strain Wood 46 than those present in the wall of Cowa