The methods of fixation and preparation of lymphoid tissues for the immunoenzyme technique are reviewed. For this technique an enzyme is used first as an antigen and then as a marker to demonstrate its specific antibody. A variety of commonly employed fixatives satisfactorily conserve tissues for the light microscopic detection of antibody but, for electron microscopy, glutaraldehyde or formaldehyde or both are the fixatives of choice. The main technical problem for electron microscopy is to reduce the size of the tissue fragments sufficiently so that the enzymes and their substrates permeate through the fixed tissues. The merits and short-comings of the different preparative techniques are examined and it is shown that the most reproducible results are obtained with 40 μm frozen sections. Some of the problems of non-specific staining arising from fixation procedures, as well as endogenous enzyme activity, are discussed. The evidence for and against antibody inactivation by fixation and enzyme inactivation by interaction with its specific antibody is reviewed.