Carboxymethylated sperm-specific lactate dehydrogenase isozyme C4 (LDH-C4) proteins from mouse and rat testes were cleaved with cyanogen bromide and trypsin. Proteins were also citraconylated and digested with trypsin. In the case of mouse LDH-C4 isozyme, all 7 CNBr and 11 limited tryptic (arginine) peptides were isolated and sequenced. Some of the CNBr peptides were further fragmented with trypsin and chymotrypsin and their compositions and/or sequences characterized. Also, 34 of the 36 expected tryptic peptides were purified, and their compositions and sequences determined. Amino acid sequences of these peptides purified from mouse LDH-C4 were overlapped into a complete covalent structure of the 330 residues. For rat LDH-C4, 5 of 6 expected CNBr peptides, 5 of 8 expected arginine peptides, and 28 of the 34 expected tryptic peptides were isolated, and their compositions and sequences were determined. Some of the CNBr and arginine peptides were further fragmented with chymotrypsin, thermolysin, or V8 protease, and their compositions and/or sequences characterized. The amino acid sequence of 85% of the 330 residues from rat LDH-C subunit has been unambiguously determined, and the sequences of the remaining regions were tentatively aligned on the basis of peptide compositions and sequence homologies with the other known lactate dehydrogenase sequences, including mouse LDH-C. A comparison of the proposed rat LDH-C sequence with the complete covalent structure of mouse LDH-C indicates that 27 differences are located in the established rat LDH-C sequence of 280 residues and that 5 additional differences are in the tentative sequence of the remaining 50 amino acids.