A new CD4-positive HeLa cell line (clone 1022) with increased sensitivity for human immunodeficiency virus (HIV) isolates derived from AIDS patients could titer infectivity of HIV from most isolates at a level equal to that observed using normal human phytohemagglutinin (PHA)-stimulated lymphocyte cultures. By use of this clone with a focal immunoassay (FIA), peripheral blood mononuclear cells (PBMC) producing HIV were detected in 15% of seropositive asymptomatic patients and 23% of AIDS patients at a frequency of 1 in 2 x 10(4) to 3 x 10(6) PBMC. HIV detection by primary FIA correlated with low CD4-positive cells counts. HIV activation in cocultures with PHA blasts resulted in increasing numbers of cells releasing HIV starting 3-4 days after cocultivation. The low incidence of HIV detection by direct FIA compared with the high incidence of HIV isolation after cocultivation with PHA blasts provided quantitative infectivity data suggesting that HIV was in a state of latency or low expression in most PBMC of AIDS patients.