This pen will write clearly under water or on wet surfaces, such as marking slots on wet Southern, Northern and Western blots. Use it for marking alignment dots on blotting filters placed containing bacterial colonies or phage plaques. Mark lanes on wet blots 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°. This pen is extremely useful for marking filters during processing under water-based buffers. The pen can also be used to write on dry paper as a standard ball-point pen.