The amino acid composition of glyceraldehyde-3-phosphate dehydrogenase isolated from rat skeletal muscle was determined. Tryptic peptide maps of the carboxymethylated protein were compared with those of the corresponding derivative of the rabbit muscle dehydrogenase. Evidence from the amino acid analysis, N-terminal amino acid and the position of the peptides in two-dimensional chromatography-electrophoresis suggests a high degree of homology between the two enzymes. However, the rat muscle glyceraldehyde-3-phosphate dehydrogenase is characterised by a markedly lower histidine content. Some differences are also revealed in the peptide maps of the rat and rabbit dehydrogenases. Four cysteine residues per subunit of rat apoenzyme may be carboxymethylated in the absence of denaturing agents (pH 8,4 75 min). Two of them are modified at pH8,0 within 30 min and are both found in the peptide, which has been demonstrated to contain the most reactive cysteine residue. This suggests cysteine residue N 153 to be the second in order of reactivity towards iodacetate. No difference in the accessibility of SH groups to iodacetic acid has been found between apoenzyme samples incubated in 0,15 M NaCl at 20 degrees and 4 degrees respectively. This indicates that enzyme inactivation caused by dissociation which occurs at low temperature brings about no measurable alterations in the SH group reactivity as compared to that observed at 20 degrees.