The use of blochemical fractionation, immunofluorescence laser-scanning confocal microscopy, and immunoelectron microscopy with mouse anti-human bcl-2 monoclonal antibody to analyze the subcellular localization of the bcl-2 gene product revealed the protein prominently in the nuclear envelope, endoplasmic reticulum membrane, and mitochondrial membranes. Electron microscopy at high magnification more precisely localized bcl-2 to the nuclear outer membrane as confirmed by the biochemical fractionation, as well as to mitochondrial outer and, to a lesser degree, inner membrane. This multisite membrane distribution of bcl-2 suggests an important role for this protein in several different membrane compartments.