The Semliki Forest virus (SFV) spike subunits p62 and E1 are made from a common coding unit in the order p62-E1. The proteins are separated by the host signal peptidase upon translocation into the endoplasmic reticulum (ER). Shortly thereafter, p62 and E1 form heterodimers. Heterodimerization preferentially occurs between subunits derived from the same translation product, so-called cis heterodimerization. As the p62 protein has the capacity to leave the ER in the absence of E1, it has been postulated that there exists a retention mechanism for the p62 protein, putatively at or near the translocon, in the ER in order to promote cis heterodimerization (B. U. Barth and H. Garoff, J. Virol. 71:7857-7865, 1997). Here we show that there exists such a mechanism, that it is at least in part mediated by the ER chaperones calnexin and calreticulin, and that the retention is important for efficient cis heterodimerization.