Temperature-sensitive mutants of an Escherichia coli K-12 strain PA3092 have been isolated following mutagenesis with nitrosoguanidine, and their ribosomal proteins analyzed by two-dimensional gel electrophoresis. This method was found to be very efficient in obtaining mutants with various structural alterations in ribosomal proteins. Thus a total of some 160 mutants with alterations in 41 different ribosomal proteins have so far been isolated. By characterizing these mutants, we could isolate not only those mutants with alterations in the structural genes for various ribosomal proteins, but also those with impairments in the modification of proteins S5, S18 and L12. Furthermore, a mutant has been obtained which apparently lacks the protein S20 (L26) with a concomitant reduction to a great extent of the polypeptide synthetic activity of the small subunit. The usefulness of these mutants in establishing the genetic architecture of the genes coding for the ribosomal proteins and their modifiers is discussed.