Ultrathin frozen sections can be cut smoothly from many fixed and appropriately treated specimens. To use such sections for immunochemical localization of intracellular antigens, fixa ion conditions must be selected to optimize at least three variables, namely, preservation of ultrastructure, preservation of antigenicity and retention of accessibility of the antigen to the antibody. Furthermore, staining of the sections must be such that both the immunolabels and structures are clearly recognized. Our efforts to attain these goals are described in relation to their historical background. Although there are still problems to be solved and improvements to be made, we now consider that cryoultramicrotomy has reached the stage of being useful in studying many questions which will not be easily approached otherwise.