Human and simian T-lymphocytes from primary PBMCs can be transformed to continuous growth by aγ2 herpesvirus, Herpesvirus saimiri (HVS) strain C488. HVS-immortalisation of human CD4⁺ T-cells (HVS T- cells) selects for a sub-population of lymphocytes that is targeted by HIV-1 in vivo; activated memory cells (CD45RO⁺, CD26^hi), which also express HIV coreceptors and CD7. This manuscript reviews recent research showing that HVS T-cells are a better tissue culture system than PBMCs for the isolation, propagation and characterisation of HIV-1, HIV-2 and SIV. HVS T-cells support the replication of both T-cell line adapted and primary viruses. The course of infection is similar to that in PBMCs but HVS T-cells are more efficient for the isolation of R5-strains that predominate in virus transmission. Although viruses isolated in the two culture systems have similar phenotypes, virus titres in HVS T-cells are higher than those produced in PBMCs. Sequence analysis of entire HIV-1 envelope genes from isolates made in both cell systems shows that cell-associated selection pressures are low and comparable. The associated gpl60 sequence changes in HVS T-cell isolates are neither more numerous nor of greater biological significance than those observed in their PBMC counterparts. HVS T-cells retain much of the biology of primary PBMCs, unlike other transformed T-cell lines. This, linked with the facts that HVS T-cells are phenotypically stable and technically less demanding than PBMCs make them good hosts for assaying virus phenotype, neutralisation sensitivity and cytotoxic T-cell activity in vitro. The HVS T-cell model is also important in other virological studies. CD8⁺ HVS T-cell virus-producing clones established from HIV-1-infected individuals are being used to study CD8-tropic isolates of HIV-1. CD4⁺ and CD8⁺ HVS T-cell clones may also provide a model for investigating non-cytolytic suppression of primate immunodeficiency viruses by unidentified soluble factors since the secretion of these is comparable in primary and HVS-transformed T-cells.