Oligodesoxyribonucleotide-directed cleavage of protein-deficient Thermus thermophilus derivatives of the 30S ribosomal subunits with RNase H is described. A homogeneous RNP fragment has been isolated as a result of the cleavage and subsequent purification in the sucrose gradient. It corresponds to the central and 5' domains of the 30S ribosomal subunit. The high compactness of the fragment in solution suggests that it can be considered as a 'beheaded' derivative of the 30S ribosomal subunit. The absence of a reconstitution stage in isolation of the 22S RNP fragment provides for its preparation in large amounts.