The relationship between plasma human immunodeficiency virus type 1 (HIV-1) infectious titer, determined by quantitative fivefold end-point dilution culture, and the detection of genomic HIV-1 RNA by immunocapture-cDNA-polymerase chain reaction was determined. The optimal plasma specimen collection and storage conditions for the use of such virologic markers for clinical trials were also determined. The variabilities in the measurement of infectious HIVLAI titer associated with intra- and interdonor peripheral blood mononuclear cells were 1.2 and 0.86 log10 50% tissue culture infective doses (TCID50)/ml (95% confidence interval range), respectively. Plasma HIV-1 titers did not change significantly after storing whole blood for 6 h either at 4 degrees C or ambient temperature or plasma for a median of 267 days (range, 259 to 482) at -70 degrees C. The detection of genomic HIV-1 RNA encapsulated in viral particles was very consistent, reproducible, and unaffected by either heparin or acid citrate or by multiple freeze-thawing. The HIV-1 RNA titers also appeared to generally correlate with the biologic titer obtained by the microculture assay. The consistency in infectious HIV-1 titer was evaluated by using 27 unfrozen plasma specimens collected from five subjects over 1 to 9 days. The median change in HIV-1 titer relative to baseline was -0.5 log10 TCID50/ml (interquartile range, -1.03 to 0.175 log10). In contrast, no significant change in HIV-1 RNA for the same frozen plasma specimens was noted. As such, immunocapture-cDNA-polymerase chain reaction may be a useful measure of plasma viremia for studying the natural history of HIV disease and assessing response to therapy.