Chemically reactive carbocyanine dyes that are intramolecularly crosslinked between the 1-position and 3′-position, their bioconjugates and their uses are described. 1,3′-crosslinked carbocyanines are superior to those of conjugates of spectrally similar 1,1′-crosslinked or non-crosslinked dyes. The invention includes derivative compounds having one or more benzo nitrogens.