N-[[(Iodoacetyl)amino]ethyl]-5-naphthylamine-1-sulfonic acid (IAEDANS) is a fluorescent reagent which reacts covalently with the free thiol groups of proteins. When the reagent is reacted with the Escherichia coli ribosome under mild conditions, gel electrophoresis shows modification of predominantly two proteins, S18 and L31', which become labeled to an equal extent. When the native (i.e., untreated) ribosome is dissociated into 30S and 50S subunits, only the 30S ribosomal protein S18 reacts with IAEDANS despite the fact that L31' is still present on the large subunit. Upon heat activation of the subunits, a procedure which alters subunit conformation, S18 plus a number of higher molecular weight proteins is modified, but not L31'; the latter reacts with IAEDANS only in the 70S ribosome or when it is free. In contrast to the relatively stable association of L31' with native or with dissociated ribosomes, dissociation of N-[(acetylamino)ethyl]-5-naphthylaminesulfonic acid (AEDANS)-treated ribosomes weakens the AEDANS-L31'/ribosome interaction, resulting, upon gel filtration analysis, in ribosomes devoid of this derivatized protein.