Abstract: A single use pen-like marking device is disclosed which is useful for marking skin to form marks which persist for a least a week and which are invisible under ambient light but fluoresce under ultraviolet light and to reveal cracks and caries in teeth which are difficult to see under normal light and for visualizing plaque on teeth without leaving a stain visible under normal light. It has a flexible housing with a dry wick-type marking nib in one end which communicates with the interior of the housing and which is impregnated with a skin fluorescing furanone. Inside the housing is a sealed fragile glass tube therein which contains a solvent for the furanone. The marking device is activated by bending the housing until the glass tube breaks and releases the solvent, which saturates the marking nib and dissolves the furanone therein.