To study host gene activation by retroviral promotor insertion, a polymerase chain reaction (PCR) assay was developed. This method allows a sensitive and selective detection of chimeric provirus-host gene transcripts, hallmarks of insertional activation events, which does not rely on an induction of tumor cell growth. We analysed HIV-1 infected cells of a CD4+ T-cell line (H9), infected peripheral blood mononuclear cells and cells in broncho-alveolar washes of AIDS patients. In each case, a variety of chimeric mRNA molecules were detected using a PCR amplification reaction and 5' primers specific to the HIV-1 LTR and 3' primers specific to poly A of mRNA. In infected H9 lymphocytes, a mRNA was identified encoding a putative protein of 145 amino-acids that was not expressed in uninfected H9 cells. This shows for the first time that HIV-1 can activate transcription of host cellular genes by promotor insertion in a fashion similar to slow-transforming avian and murine retroviruses.