The total protein mixture of the 30S subunit (TP-30) of the bacterium Thermus thermophilus has been purified using reverse-phase HPLC and the proteins obtained were identified both by means of two-dimensional polyacrylamide gel electrophoresis as well as by amino-terminal amino acid microsequence analysis. The proteins are numbered according to their primary structural similarity with known prokaryotic ribosomal proteins. Eight of them, namely proteins S6, S7, S9, S10, S14, S15, S16 and S17 run at different positions in the two-dimensional gel electrophoresis system to those suggested [Sedelnikova, S. C., Agalarov, M. B., Garber, M. & Yusupov, M. M. (1987) FEBS Lett. 220, 227-230]. All characterized proteins are homologous to known ribosomal proteins from other species, except for a small basic protein which shows homology only to a ribosomal protein from spinach chloroplasts [Choli, T., Franceschi, F., Yonath, A. & Wittmann-Leibold, B. (1993) Biol. Chem. Hoppe-Seyler 374, 377-383; Subramanian, A.-R. (1984) Trends Biochem. Sci. 9, 491-494].