Three procedures for fluorescence assay of hydroxynaphthoquinones are reported. The first procedure, based on prior sodium dithionite reduction and determination of the fluorophore in butyl acetate, is applicable to all of the hydroxynaphthoquinones investigated (detection limit: +/- 0.020 microgram of quinone/g of butyl acetate). The second method, specific for 5-hydroxy-1,4-naphthoquinones (juglone series), involves warm reduction with stannous chloride in an acid medium and determination of the fluorophore in chloroform (detection limit: +/- 0.020 microgram of quinone/g of chloroform). The third method, based on the reaction of Guilbault and Kramer, is applicable to 5-hydroxy- and 5,8-dihydroxy-1,4-naphthoquinones having both free quinonoid positions (detection limit: +/- 0.020 microgram of juglone/g of dimethyl sulfoxide and 0.070 microgram of p-naphthazarin/g of dimethyl sulfoxide). The nature of fluorophores also was investigated.