A method is described for the fixation of blocks of tissue for use in studies employing immunofluorescence. This method consists of fixing thin blocks in 95% ethanol and carrying out the subsequent dehydration and clearing at refrigerator temperatures (4°C). Thereafter, embedding in paraffin and sectioning by the standard microtomy is easy. This method results in preparations which are histologically more precise in the localization of antigen or antibody than preparations of frozen tissues; and with rabbit antibody and bovine serum albumin, the sensitivity of detection is enhanced. Bovine serum albumin can be found for longer periods after injection than is possible with frozen sections. Other antigens for which this procedure has proved satisfactory are bovine gamma globulin, horse ferritin, influenza A virus, diphtheria and tetanus toxoids. Hen's ovalbumin deteriorated. New antigens or new antibodies should be tested before being committed to this method.