Periplasmic hydrogenase [hydrogen:ferricytochrome c3 oxidoreductase, EC 1.12.2.1] from Desulfovibrio vulgaris Miyazaki K (MK) was purified to homogeneity. Its chemical and immunological properties were examined and compared with those of other Desulfovibrio hydrogenases. The pure enzyme showed a specific activity of 1,000 mumol H2 evolution min-1 (mg protein)-1. The enzyme had a molecular weight of 50,000 as estimated by gel filtration and consisted of a single polypeptide chain. The absorption spectrum of the enzyme was characteristic of an iron-sulfur protein and the extinction coefficients at 400 and 280 nm were 34 and 104 mM-1. cm-1, respectively. It contained 9.4 mol iron and 6.9 mol of acid-labile sulfide per mol. The amino acid composition of the preparation was very similar to the value reported for D. desulfuricans NRC 49001 hydrogenase. Rabbit antisera were prepared against the enzyme of D. vulgaris MK. Ouchterlony double diffusion and immunotitration tests of crude extracts from several strains of Desulfovibrio revealed that the enzyme from MK cells was immunologically identical with those from D. vulgaris Hildenborough and D. desulfuricans NRC 49001, but different from those from D. vulgaris Miyazaki F (MF) and Miyazaki Y, and D. desulfuricans Essex 6 strains. It is concluded that among Desulfovibrio hydrogenases, those from D. vulgaris MK, D. vulgaris Hildenborough and D. desulfuricans NRC 49001 form one group in terms of both subunit structure and antigenicity.