Free virus (virus not present within cells) was detected in the plasma of all human immunodeficiency virus (HIV)-infected individuals studied. Plasma samples from asymptomatic individuals and individuals with HIV disease were tested. The levels of virus varied, but high virus titers correlated directly with HIV-related symptoms and low CD4+ lymphocyte counts. Effective detection of infectious virus depended on the use of an enzyme-linked immunosorbent assay for p24 core antigen and culture conditions in which plasma was added to mitogen-stimulated lymphocytes within 3 h of venipuncture. When there were delays in the time to culturing of plasma, neutralizing antibodies and perhaps other factors present in the plasma were found to reduce the efficiency of virus recovery. Plasma stored at -70 degrees C for several months maintained a stable level of free virus. These results suggest that measurement of HIV present in plasma under optimal conditions could be an efficient way of monitoring the clinical state of an individual and the effects of antiviral therapy.