Fearon''s reaction for -S-S-, =C=S and non-dissociated hydrazine and amidine groups was adapted to the detection of sub-stances on paper. Paper chromatograms were developed with butanol-acetic acid-water and then sprayed with a solution containing 0.75% sodium pentacyanoaminoferrate (11) and one drop concentrated H2O2 in 4% sodium hydroxide. Rf values and the colors produced are listed for: biguanidine, arginine, agmatine, arcaine, creatine, glycocyamine, dicyandiamidine, guanidine, formoguanamine, urea, methyl isothio urea, thiourea, dicyanamide, sulfaguanidine, and p-chlorophenyl-biguanidine. Minimum detectable quantities are also given.