This unusual pen will write clearly on wet surfaces, such as marking slots on wet Southern, Northern, and Western blots Use it for marking alignment dots on blotting filters placed over agar plates containing bacterial colonies or phage plaques. Mark lanes on wet plots prior to cutting the filter into strips to be incubated separately with different antisera or probes. It can be used to correct or write on wet labels that were kept in a water bath, in liquid nitrogen or in a freezer at 0 degrees. The Wet Surface Pen is extremely useful for marking filters during processing under water-based buffers. The pen can, of course, also can be used to write on dry paper as a standard ball pen.