The reaction of N-(3-pyrene)maleimide with thiol groups of rabbit reticulocyte ribosomes offers a possible fluorescent probe for studying ribosomal structure and conformation. At relatively low concentrations of N-(3-pyrene)maleimide a group of 30-40 readily reactive sulfhydryl residues is derivatized. The major ribosomal proteins containing these thiol groups are identified as S2 + S3, S5, S7, S8, S29, S31, S32, L1, L5, L6, L10 + L14, L15, L18 + L19, and L36. Ribosomal activity, as measured by the nonenzymic binding of phenylalanyl-tRNA and polyphenylalanine synthesis, is inhibited by this degree of reaction with N-(3-pyrene)maleimide. The inhibition is relieved by the prior binding of polyuridylic acid to the ribosomes while the extent of derivatization by N-(3-pyrene)-maleimide is diminished only slightly. The average relative polarization of the fluorescence of the ribosomal bound N-(3-pyrene)maleimide changes significantly with the degree of derivatization of ribosomal thiol groups or with the binding of polyuridylic acid, indicating the value of such a fluorescent thiol-derivatizing agent as a probe of ribosomal structure.