A simple, rapid, quantitative syncytium-forming microassay for the detection of human immunodeficiency virus (HIV-I) isolates is described. A virus-syncytial sensitive clone of CEM cells (CEM-SS) was identified and made adherent to flat bottom 96-well microtiter dishes. Following the addition of virus, these cells develop easily quantifiable, adherent syncytia on a background of confluent, normal CEM-SS monolayer in 4 to 6 days. One-hit kinetics for syncytia formation were obtained at various multiplicities of infection. Syncytia are associated with complete virion production and cytoplasmic localization of the p24 core protein (detected by immunofluorescence). Total infectious virus can be accurately determined in this assay; these results showed a close correlation with p24 and gp120 induction when microtiter well supernatants were passed to fresh cells and evaluated by competitive radioimmunoassay. Studies of p24 antigen induction at and beyond the end point of syncytia formation indicate that there are no detectable nonsyncytial variants in standard HIV-I stocks. Six divergent HIV-I isolates (HTLV-IIIB, -RFII, -MN, -RUTZ, -CC, and LAV-1), as well as HTLV-IIIB and LAV-1 reisolated from persistently infected chimpanzees, produce quantifiable syncytia which vary slightly in their developmental morphology. Accurate neutralization titers are readily obtained from easily constructed multiplicity curves derived from serial dilutions of test sera. Inherent within this system is a flexible method for studying various kinetics of antibody/virus interactions, as well as blocking and interference studies with any candidate antiviral compounds.