Immunoelectronmicroscopy was employed to investigate the ultrastructural localization of the low density lipoprotein (LDL) receptor and the protein moiety of LDL (apo B) on normal human epidermis in situ and reconstructed human epidermis in vitro. For this purpose ultrathin sections of Lowicryl K4M embedded material were incubated with monoclonal antibodies against the LDL receptor and apo B followed by a second antibody conjugated to 15 nm colloidal gold. Examination of the sections revealed a similar distribution of the LDL receptor and apo B in normal human epidermis and in reconstructed epidermis. In both systems the amount of LDL receptors decreases during keratinocyte differentiation. In contrast, apo B molecules are more abundantly expressed in the upper layers of the stratum spinosum and the stratum granulosum. The great similarities in the distribution of the LDL receptor and apo B between the in vitro reconstructed epidermis and its in vivo counterpart provide additional proof that reconstructed epidermis is an excellent tool to investigate proliferation and differentiation processes of keratinocytes in vitro.