A low-temperature embedding technique using the polar resin Lowicryl K4M with protein A gold as a marker was utilized to localize a variety of antigens at the ultrastructural level. Carcinoembryonic antigen immunoreactivity was noted over the microvilli, secretory vacuoles, and the Golgi apparatus of normal colonic epithelium. Epithelial membrane antigen was localized to the luminal plasma membrane and cytoplasmic vesicles of glandular epithelium. The lymphoid antigens leucocyte common antigen and HLA-DR were both found on the membranes of lymphocytes and histiocytes. In addition, HLA-DR immunoreactivity was noted in the Golgi apparatus of these cells. The method described appears suitable for the localization of both surface and intracellular antigens with excellent preservation of both morphology and antigenicity.