PAP pens are useful in immunostaining applications that use small amounts of valuable antiserum. These pens create a water repellent barrier around specimens or sections, retaining antiserum within the defined area and limiting the amount of antibody required. Multiple sections, separated by individual circular barriers, may be applied to the same slide for easy comparison. The PAP pen liquid is formulated to be insoluble in alcohol and acetone, and stable for long time periods. It can be removed by xylene if desired after staining is completed. Slides must be clean before the barrier is applied and can be coated with egg albumen, glycerine or poly-L-lysine if required.