Ribophorin I and II (RPN I and RPN II), two specific glycoproteins, span the rough regions of the endoplasmic reticulum (RER) and are thought to play an important role either in translocation or in the maintenance of RER. Studies with human-mouse somatic cell hybrids have localized the gene for RPN I on human chromosome 3q, while RPN II is on chromosome 20. Using a radioactive labelled cDNA probe, we have regionally mapped the RPN II gene to human chromosome 20q12-q13.1 by in situ hybridization. This assignment predicts a location of the murine homologue, Rpn-2, to the syntenic segment on mouse chromosome 2 in close proximity to Ada, Src and Gnas.