At the end of the exponential growth phase the Gram-positive bacterium Clostridium acetobutylicum performs a metabolic switch from classical sugar fermentation accompanied by the production of acetate and butyrate to reinternalization and oxidation of these acids to acetone and butanol. Protein synthesis in acidogenic and solventogenic C. acetobutylicum cells was compared by two-dimensional gel electrophoresis of radioactively labeled proteins. The results show that the switch from acid to solvent production is accompanied by dramatic changes in the protein pattern. During solventogenesis, the synthesis of 52 proteins out of 130 analyzed was increased more than twofold, the synthesis of 34 proteins decreased to less than half as compared with synthesis in acidogenic cells. The changes in protein synthesis were generally reflected by changes in the abundance of the respective proteins, as determined from quantitative analysis of silver-stained second-dimensional gels. Nine proteins induced during solventogenesis were identified by N-terminal microsequencing. One of these proteins, the acetoacetate decarboxylase, is directly involved in solventogenesis. Other proteins synthesized in higher amounts during solventogenesis were three general stress proteins (DnaK, GroEL, Hsp 18), two enzymes involved in serine biosynthesis (serine aminotransferase and 3-phosphoglycerate dehydrogenase) as well as a seryl-tRNA synthetase. mRNA analysis provided evidence that the latter three are encoded by genes organized in an operon and are transcriptionally induced at the onset of solventogenesis. The proteins acetoacetate decarboxylase and Hsp 18 occurred in two variants, indicating possible covalent modification of these proteins.