A series of 14 3-formylrifamycin SV N-(4-substituted phenyl)piperazinoacethydrazones has been synthesized and evaluated for their antimicrobial activity. The compounds were found active against Bacillus subtilis, Staphylococcus aureus, Mycobacterium phlei, and Mycobacterium tuberculosis but not as active as rifampin. The compounds also exhibited significant activity against Clostridium perfringens and in this bacterial system some were more active than rifampin. The QSAR showed that the activity against B. subtilis depended only on lipophilicity, and the regression equation was linear. A parabolic relationship between the antibacterial activity and lipophilicity of the compounds was found in Staph. aureus. Additionally, the activity was dependent upon the electronic and steric effects of the phenyl substituents. The sensitivity of M. phlei to the compounds was found to correlate well with a linear combination of hydrophobic, electronic, and steric parameters. No statistically significant correlation was possible between the physicochemical parameters studied and the activity of the compounds against C. perfringens and M. tuberculosis.