IFN-γ increases susceptibility to HIV-1IIIB infection in U937 cells. Standardized PCR amplification of LTR(U3)-(U5)LTR circular forms of HIV-1 DNA was applied to assess de novo viral replication,23 20 hours after exposure. Samples were normalized by enumerating viable cells using trypan blue exclusion. β-Globin amplification was applied to confirm comparability of the samples. Results are representative of 4 independent experiments.