A detailed procedure is described that allows detection of the presence of human immunodeficiency virus-1 (HIV-1) transcripts within both acetone-fixed tissues and peripheral blood mononuclear cells. This assay uses cDNA probes labelled by a non-isotopic procedure that results in the modification of cytosine residues through covalent linkage to a sulphone group. In situ hybridized probe is then detected by an alkaline phosphatase-conjugated antibody specifically directed against the sulphone hapten. This procedure is specific, rapid and safe and can be applied in the research as well as in the clinical pathology settings.