The endoplasmic reticulum (ER) contains a family of luminal proteins (reticuloplasmins) that are normally excluded from the secretory pathway. However, reticuloplasmins are efficiently secreted when murine fibtoblasts are treated with calcium ionophores. The secreted and cellular forms of endoplasmin ate clearly distinguishable on the basis of gel mobility and endoglycosidase H sensitivity. Reticuloplasmin secretion leads to the depletion of the proteins from the ER and their accumulation in the Golgi apparatus. The stress response to calcium ionophore induces reaccumulation of reticuloplasmins in the ER and sup presses their secretion. Secretion is also associated with changes in the structure and distribution of the ER. These observations show that perturbation of cellular calcium levels leads to the breakdown of the mechanism for ER retention of teticuloplasmlns and suggest a role for calcium ions in their sorting from secretory proteins.