Measurements of human immunodeficiency virus by quantitative RNA and DNA polymerase chain reaction (PCR), cell and plasma infectivity dilution cultures, and immune complex-disassociated p24 antigen-capture ELISA were made repeatedly in 10 subjects receiving long-term zidovudine treatment before and after therapy was changed to didanosine. Comparison of baseline assays showed that quantitative cell cultures, plasma RNA, and proviral DNA were measurable in all subjects and that cell culture results were significantly correlated with measures of nucleic acids. Plasma viremia (as indicated by culture) and p24 antigen were detected in three measurements in 3 of 8 and 6 of 10 subjects, respectively. Significant decreases in plasma RNA and cell dilution cultures from baseline were maintained for up to 6 months after initiation of didanosine therapy. These findings demonstrate a decrease in virus burden with the use of didanosine; however, continued detection of plasma RNA suggests that additional antiviral therapy will be required to suppress viral replication.