Ninhydrin derivatives bearing sulfur-containing groups at position 5 exhibit excellent properties as fluorogenic fingerprint reagents on paper. Their sensitivity markedly exceeds that of ninhydrin and even that of 5-methoxy ninhydrin. Preliminary experiments indicate it as equal to or better than 1-8- diazafluorene-9-one (DFO). One member of this group, 5-methylthio ninhydrin, has been recently declared an operational fingerprint developer in our laboratory.