Abstract Isolation of ribosomal precursors from Escherichia coli K12 is described. The RNA and protein content of the precursor particles was determined. One physiologically stable precursor was found for the 30 S subunit. The assembly scheme is as follows: p16 S RNA + 9 proteins → p30 S (“21 S” precursor) p30 S + 12 proteins → 30 S subunit where p is precursor. Each of the two precursors for the 50 S subunit, P 1 50 S and p 2 50 S (“32 S” and “43 S” precursors, respectively), contains p5 S + p23 S RNA's in a 1:1 molar ratio. The assembly scheme is as follows: p23 S RNA + p5 S RNA + 16 or 17 proteins → p 1 50 S In contrast to the p 2 50 S precursor the p 1 50 S precursor is not similar to any core particles, which were obtained by treating 50 S subunits with different concentrations of LiCl or CsCl. The precursors p30 S and p 2 50 S can be converted into active 30 S and 50 S sub-units, respectively, by incubation at 42 °C in the presence of ribosomal proteins and under RNA methylating conditions.