The amino acid composition and amino-terminal sequence have been determined for the alanine-rich, acidic ribosomal 'A' protein (equivalent to Escherichia coli L7/L12) from three procaryotic cell types that live under extreme environmental conditions (Arthrobacter glacialis, Clostridium pasteurianum, and Bacillus stearothermophilus) as well as from wheat germ, a eucaryote source. These data are compared with previously published 'A' protein sequences from other procaryotes and eucaryotes. All the procaryotic 'A' proteins, with the exception of the very acidic 'A' protein from Halobacterium cutirubrum, show similar charge, size, and amino acid composition, as well as an extensive sequence homology in the N-terminal region. Some differences are observed between gram-negative and gram-positive bacteria. The 'A' proteins from eucaryotes contain two tyrosine molecules, an amino acid absent in procaryotic 'A' proteins, as well as a reduced number of valine residues and an increased amount of aspartic acid. The N-terminal sequence of wheat germ 'A' protein shows considerable homology with other eucaryotic 'A' proteins and also with H. cutirubrum. It also shows some sequence homology with E. coli 'A' proteins.