The whole cell proteins and the ribosomal proteins of Mycoplasma capricolum ATCC 27343 have been analyzed by two-dimensional polyacrylamide gel electrophoresis. The M. capricolum cell is relatively rich in basic proteins. The number of total protein spots detected was approximately 355, which is less than one-third of that of Escherichia coli or Bacillus subtilis. In contrast, the number (30 and 20 protein species have been found to be present in the 50S and 30S ribosomal subunits, respectively) and the size of the ribosomal proteins in the M. capricolum do not seem to be significantly different from those of typical eubacteria.