We have characterized the maturation of DV1 RSPs produced by the HeLa-prME cell line. Sucrose gradient and sedimentation analysis demonstrated that the DV1 RSPs are concentrated in fractions containing 20-30% sucrose and are sensitive to detergent treatment. Although prM/E heterodimers were not found in our experiments, possibly because prM/E interaction is weak, we observed processing of prM into M protein in HeLa-prME cells. This cleavage was sensitive to NH 4 Cl treatment, which inhibits acidification of the trans-Golgi compartment and, hence, the activity of furin protease. Newly synthesized E proteins first form heterodimers with prM proteins in the ER of host cells and then rearrange as homodimers during the process of secretion [37, 38] . The rearrangement from heterodimer to homodimer was first thought to require cleavage of prM by furin in the trans-Golgi to form M and soluble pr proteins [39] . A recent study, however, has shown that such rearrangement is mainly caused by the progressive acidification of the milieu along the secretory pathway, which facilitates prM cleavage by furin [6, 10] . If rearrangement is a pre-requisite for prM cleavage to occur, it could explain, at least in part, why E homodimers were still observed in our study following treatment with the acidotropic reagent NH 4 Cl. The precise mechanism underlying their presence in RSPs under these experimental conditions, however, requires further investigation. Finally, we found by pulse-chase experiments that 8 hours are necessary for production and secretion of RSPs. Altogether, these results show that mature DV1 RSPs are efficiently produced by the stable HeLa-prME cell line.