Nucleic acids may be labeled by intercalating the alkylating intercalation moiety of a labeling reagent into a partially double-stranded nucleic acid to form a complex and activating the complex to cause covalent bonding between the reagent and the nucleic acid. Preferably, the labeled nucleic acid is a hybridization probe for detecting nucleic acid sequences capable of hybridizing with a hybridizing region of the nucleic acid. Also preferably the label moiety is non-radioactive. The labeling reagent is of the formula:[A--[B--Lwhere A is an alkylating intercalation moiety, B is a divalent organic moiety of the formula: ##STR1## where Y is O, NH or N--CHO, x is a number from 1 to 4, y is a number from 2 to 4, and L is a monovalent label moiety, wherein B is exclusive of any portion of the intercalation and label moieties.Preferably A is a 4-methylene-substituted psoralen moiety, and most preferably A is a 4'-methylene-substituted-4,5',8-trimethylpsoralen moeity and L is biotin.