Rhodamine conjugates:specific and nonspecific binding properties in immunohistochemistry.

Anionic-exchange fractions of IgG labeled with FITC, MRITC, RB200SC, or RBITC were tested on different substrates, and the resultant fluorescence was evaluated with the Ploem optical system. Conjugations with MRITC or RB200SC were found to afford the following advantages over FITC: immunofluorescence sensitivity was elevated six to seven times on a molar basis; high sensitivity could be combined with a wide specificity interval; there was negligible fading of emitted light; there was negligible tissue autofluorescence at the excitation wavelength (546 nm); because of the two latter points, repeated observations could be made on tissue sections stored for several years; and eosinophilic leukocytes that are prone to yield nonspecific staining could easily by identified by switching to ultraviolet-blue light excitation.