The thermostability in vitro of dimeric and tetrameric malate dehydrogenases [S)-malate:NAD+ oxidoreductase, EC 1.1.1.37) from mesophilic and thermophilic bacteria shows a good correlation to the growth temperature of the source organism but no consistent relationship to enzyme subunit structure. The thermophile malate dehydrogenases are, in general, more resistant to the surfactants, sodium dodecyl sulphate (SDS) and hexadecyltrimethylammonium bromide, and to the denaturants, guanidinium chloride and urea, than their mesophilic counterparts, with the dimer in each thermal class being more resistant to the chemical perturbants than the tetramer. Sedimentation analysis suggests that denaturation of the malate dehydrogenases by acid-periodate or SDS produces discrete subunits, whereas denaturation by guanidinium chloride followed by carboxymethylation yields ill-defined protein species. SDS and acid-periodate were therefore preferred to generate denatured malate dehydrogenases for use as immunogens and antigens. The native malate dehydrogenases exhibit immunological cross-reactivity only when they are in the same oligomeric form and derived from closely related species, which may, however, be from different thermal classes. Taking immunological cross-reactivity as an indicator of structural similarity, this supports the idea that the thermophilic trait evolved independently within each phyletic line. With denatured malate dehydrogenases as immunogens and antigens, cross-reactivity is manifested between all the malate dehydrogenases examined. This suggests that appreciable primary structural homology exists between the malate dehydrogenases, whether dimeric or tetrameric, from thermophiles and mesophiles and from various taxa.