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OBJECTIVE: To establish the syncytium-inducing (SI) phenotype and zidovudine (ZDV) susceptibility of HIV-1 isolates obtained from autopsy specimens. METHODS: Isolation of HIV was attempted from autopsy specimens obtained from 76 AIDS patients. Specimens of lymph node, spleen, spinal cord, brain and cerebrospinal fluid (CSF) were processed and cultured with peripheral blood mononuclear cells (PBMC) from seronegative donors. Biological phenotype was determined in a T-lymphocyte line (MT-2). ZDV susceptibility was evaluated in a PBMC-based assay. Sequencing of amino-acid codons in the reverse transcriptase gene previously shown to be associated with ZDV resistance was carried out on a subgroup of isolates. RESULTS: HIV was recovered from tissue specimens and CSF up to 5 days post-mortem, but recovery rate of infectious virus decreased as the time between autopsy and specimen processing increased. There was a lack of concordance between PBMC isolates and isolates from different tissue sites with respect to SI phenotype. ZDV-resistant virus was isolated from post-mortem specimens of patients who had received long-term ZDV therapy up until or shortly before their death. ZDV-sensitive virus re-emerged in the lymph node of patients who ceased treatment several months prior to death. Phenotypically sensitive virus obtained from lymph node tissue of three patients after a relatively short time off ZDV (4-6 months) retained some of the amino-acid substitutions known to be associated with resistance. CONCLUSION: The data suggests that ZDV resistance and re-emergence of sensitive virus does not originate in peripheral cells, and that these cells and tissues are seeded with virus present elsewhere, possibly in the germinal centres of the lymph node.