Ethane sulfonic acide hydrazide (esh: CH3CH2SO2NHNH2) derivatives as 5-methylsalicyl-aldehyde-ethanesulfonylhydrazone (5msalesh), 5-methyl-2-hydroxyacetophenoneethane sulfonylhydrazone (5mafesh) and their Ni(II), Co(II) complexes have been synthesized for the first time. The Structure of these compounds has been investigated by elemental analysis, FT-IR, H-1 NMR, C-13 NMR, LC/MS, UV-vis spectrophotometric method, magnetic Susceptibility, thermal studies and conductivity measurements. The antibacterial activities of synthesized compounds were studied against Grain positive bacteria: Staphylococcus aureus, Bacillus subtilis, Bacillus magaterium and Gram negative bacteria; Salmonella enteritidis, Escherichia coli by using the microdilution broth method. The biological activity screening showed that ligands have more activity than complexes against the tested bacteria. The inhibition activities of these compounds on carbonic anhydrase II (CA II) have been investigated by comparing IC50 and K-i values and it has been found that 5msalesh and its complexes have more enzyme inhibition efficiency than other compounds. (C) 2009 Elsevier B.V. All rights reserved.