Efficient dimerisation and encapsidation are associated with correct assembly and particle maturation in HIV-2. COS-1 cells were transfected with envelope-deleted provirus and virions were harvested 48 h later. Purified HIV-2 particles were analysed by negative staining and electron microscopy. (A) Five examples of what were assessed as immature particles (i), mature particles (ii) and particles containing multiple cores (iii) are shown. Scale bar: 100 nm. (B) The proportion of mature, immature and multiple core particles was calculated for wild type (WT) and DM mutant HIV-2 after examination of 193 WT and 199 DM particles by two individuals independently. Error bars correspond to the SD. *, Student t test p value < 0.05. (C) The particles were measured and the average sizes of WT and DM particles were reported on the graph. Error bars correspond to the SD; n=215 for WT and 189 for DM; *, Student t test p value < 0.05.