Human phosphoglyceric acid mutase comprises M-, B- and MB-type isozymes composed of the combination of the muscle-specific (M) and nonmuscle-specific (B) subunits. Human DNAs encoding M and B subunits were respectively reconstructed at their 5' regions without changing the amino acid sequences, and expressed directly in Escherichia coli under controls of the trp promoter. M- and B-type isozymes were over-produced in the bacterial cytoplasm as soluble, active forms, which have been purified and characterized. MB-type was synthesized in vitro by recombining M- and B-type. All the three recombinant isozymes thus obtained were the same in properties tested as the naturally-occurring ones. Polyclonal IgGs specific to the M-type, B-type and MB-type isozymes were prepared from rabbits immunized with the respective isozymes mainly by treating with the columns bound with the M- or B-type isozyme. A method for the immunoassay of the MB-type isozyme which exists specifically in cardiac muscle, is now under development.