The ultrastructural detection and identification of human retroviruses--HTLV (human T-cell lymphotropic virus) and HIV (human immunodeficiency virus)--have become an everyday task for pathologists and virologists as well as for cell and molecular biologists. The development of better and conventionally available immunocytochemical techniques, such as pre- or postembedding immunocytochemical methods, cryofixation-variants and low temperature embeddings, have made it possible to use them in this field. With the help of these methods the structural proteins of HTLV-I and HIV have been identified in infected cells. The virus assembly at the cell membrane has also been described in detail. Using these methods the incorporation of human transplantation antigens into the envelope of these viruses can be followed. Future studies should establish the pathological significance of this process.