Inactivation of HIV-1 contaminated materials or biological samples is of great importance and requires the use of a reliable assay to detect residual infectivity. In this study we treated cell-free or cell-associated (monocyte-derived macrophages) HIV-1 with two chemicals known for their antiviral activities, beta-propiolactone (beta PL) and formaldehyde (FO), and tested it for the presence of residual infectivity. HIV-1 infected primary monocyte-derived macrophages (MDM) or cell-free HIV-1 were fixed with increasing concentrations of either beta PL or FO for 1 day at 4 degrees C. Then either fresh primary MDM or fresh medium was added, and the supernatant p24 levels were assayed up to 12 days after infection. All the supernatants harvested were added to indicator cells, fresh primary MDM, to assess for residual infectivity. The results show that p24 measurement is not a reliable assay for the detection of residual infectious virions after chemical fixation of HIV-infected primary MDM. In contrast, the use of indicator primary cells (MDM) is a much more sensitive and reliable assay. By performing an indicator cell assay we showed that FO efficiently inactivates cell-associated and cell-free HIV-1 at concentrations as low as 1% v/v. In contrast beta PL is more efficient in inactivating cell-free than cell-associated virus and does not inactivate cell-associated HIV-1 at concentrations as high as 1% v/v.