The presence of human immunodeficiency virus (HIV, or HTLV-III) genome sequence was investigated by means of in situ hybridization in cryostat sections of lymph nodes from lymphadenopathy syndrome (LAS) patients. The technique employed involved the modification of the DNA probe by chemical insertion of an antigenic sulfone group in cytosine moieties and the visualization of DNA by a double-antibody immunohistochemical reaction. The hybrid formation was revealed in five out of ten cases: in all positive samples, HIV was mainly observed in the cytoplasm of lymph node cells. The method of in situ hybridization described in the present paper is specific and has some advantages if compared with other techniques based on the use of DNA probes labelled with radioisotopes or biotin by nick translation.